Pokémon/Useless Useful Spell

With how competitive Pokémon has become in recent years, there are several ways for players to spec their critters to make them viable in a competitive environment. While there are many tools that each Pokémon can utilize, there are some instances where they aren't very useful. This could be the result of many factors, including but not limited to:

Having abilities that don't benefit that particular species.
Learning moves that don't compliment the Pokémon's stat spread.
Abilities that can be made redundant due to typing.
Moves and abilities that don't work for how the Pokémon is meant to function.

Examples

Abilities

The most notorious example is Frillish and Jellicent having Damp as their hidden ability. Having immunity to self-destructing moves is great and all, but as they are part Ghost-type Pokémon, Frillish and Jellicent are already immune to those moves anyway. It doesn't help that the generation they were introduced in also nerfed Explosion so it no longer halves the Defense stat, making them less useful in the first place. That said though, later generations introduced the moves "Mind Blown", and even later, "Misty Explosion", which are Fire and Fairy type moves respectively, making Damp Frillish/Jellicent slightly less useless.
FromPokémon X/Y onward, Electric-type Pokémon can no longer be paralyzed by any means, which similarly makes Stunfisk's Limber redundant.
All of Rotom's forms take on Levitate as their sole ability, including Fan Rotom; the Flying type variant that is ALREADY naturally immune to Ground-type moves.
Aside from allowing the player to see the opponent's held items, the ability "Frisk" is not useful for anything else in competitive play.
Weavile gets two abilities: Pressure and its hidden ability, Pickpocket. The former doubles the opponent's PP usage, the latter lets it steal the opponent's held item when said opponent makes contact. However, Weavile's stat spread isn't suited for PP stalling (70/120/65/45/85/125), since it is meant to play as a Glass Cannon, thus it can't really afford to take a physical hit. The same thing applies to "Pickpocket", since Weavile's unimpressive 65 Defense means it doesn't want to take a physical attack. These two abilities are much better suited to Pokémon that can take some punishment.
Unnerve is an ability that prevents the foe from consuming Berries; however, Berries are rarely used during the campaign, making Unnerve not worth having. Even in competitive, most Pokémon with Unnerve have far more useful alternative abilities anyway.
Lightning Rod redirects Electric-type attacks, and boosts the Pokémon's Special Attack while also making the Pokémon immune to Electric-type attacks. However, out of the 20 Pokémon that can have this ability, only 6 of them have a Special Attack stat that is at least serviceable, while the other 14 do not.
Salazzle's signature ability, Corrosion, allows her to afflict Poison- and Steel-type Pokémon with poison; however, it doesn't allow Poison-type attacks to bypass Steel's Poison immunity, and like Weavile, Salazzle is not built to whittle down her opponents with Poison damage, due to being a Fragile Speedster.
At first Oblivious was rarely used, as its only function was to stave off Infatuation, a status that is incredibly rare. Soon averted when Oblivious became capable of blocking Taunt and, eventually Intimidate.
The popular Sheer Force is extremely potent, as it removes all secondary effects of moves to boost their damage by 30%. It also removes the recoil damage from Life Orb while retaining the item's damage boost. Unfortunately for one Pokémon, Toucannon, it learns very few moves to properly take advantage of Sheer Force.
Ledian can have the ability Iron Fist as its Hidden Ability, which boosts the power of punching moves by 20%. To be fair, Ledian can learn a handful of these moves (including the priority move Mach Punch). However, Ledian's atrocious base 35 Attack stat still gives it the power of a feather even with Iron Fist.
Orbeetle has the ability Swarm, which boosts its Bug-type moves when its HP drops below 33%. Unfortunately, Orbeetle only learns 4 Bug-type attacks (2 via level, 2 via TM/TR), and out of those 4 moves, only Bug Buzz is even useful because Infestation has poor base power, while U-Turn and Leech Life run off its poor base 45 Attack stat. Outside of that, its other Abilities are of extremely limited use (Frisk reveals the opponent's held items, Telepathy is only useful in double battles).
Some Pokémon have Abilities that are only useful while exploring the overworld, and serve no use in battle. For example, why would anyone even entertain the though of running Starmie with the ability Illuminate? An ability that only doubles the encounter rate of wild Pokémon, and nothing else.
Delibird has two main Abilities: Vital Spirit to prevent it from falling asleep, and Hustle which boosts Attack but reduces Accuracy. Its hidden ability? Insomnia, which does the exact same thing as Vital Spirit. One has to wonder why Game Freak even bothered giving Delibird a hidden ability at all. They might as well have given it Vital Spirit twice!
Merciless is the exclusive ability of Mareanie and Toxapex, which always score a critical hit if the target is poisoned. While Toxapex is built around spreading poison through Toxic, Toxic Spikes or Baneful Bunker, the issue is that Toxapex itself is a Stone Wall, meaning that it's not going to hit particularly hit even with the Mecriless boost.
Galarian Weezing has Misty Surge as its hidden ability. While a fantastic ability on its own since it summons Misty Terrain to block statuses, it contradicts Weezing's playstyle as it prefers spreading statuses through Will-o-Wisp or Toxic.

Moves

One notable aspect of Normal-type is their wide variety of Special moves... except for the fact most Normal-type Pokémon tend to have low special attack to properly take advantage of it anyway. Out of them, the Porygon line and Teraspagos are one of the few exceptions that can capitalize on their wide Special movepool. Clefable and Togekiss were former examples that can utilize their special moves well, but they have since been retconned into Fairy-type in X and Y.
Most Dark-type Pokémon before Gen III couldn't use their STAB moves effectively because of Dark being a Special type at the time. Only Houndoom and Tyranitar had a decent enough Special Attack to offset this issue, but everything else was a physical attacker. Thankfully, the Physical/Special split happened in Gen IV changed all existing Dark moves into Physical moves.
A good handful of Ghost-types, like Cofagrigus and Chandelure can learn the move Dream Eater, a powerful move with base 100 power, and HP recovery. It's just too bad that those two Pokémon don't actually learn any sleep inducing moves, though.
Shedinja has access to the move Final Gambit, a sacrificial move that does damage equal to the user's remaining HP. So naturally, it makes sense that the one Pokémon that only has 1HP can learn this move.
Electrode can learn Gyro Ball, a move that does more damage the slower the user is compared to its target. But Gyro Ball is almost useless when used by Electrode considering not just its poor Attack stat (50), but Electrode is one of the fastest Pokémon in the games with its 150 base Speed. Luckily, it learns Electro Ball as well.
On the opposite end of the spectrum, Vikavolt can learn Electro Ball via TR80 in Sword/Shield, which works in tandum with its amazing 145 Special Attack, but is greatly offset by its poor Speed stat of just 43, making the attack less potent.
Umbreon can have Synchronoise as an Egg move, which only damages opposing Pokémon the same type as the user. So you can just image what kind of damage this Psychic-type move can have when used by the Dark-type Umbreon...
Mew and Cryogonal are able to learn Attract, but can't do anything with it seeing as they are genderless.
Leech Life was a pathetically weak move that only had 20 base power. It was so weak that not even its healing properties could make it worth using. At least, until Gen VII where its power not only quadrupled, but it also had increased availability as a TM move!
The Elemental Punches were this to Hitmonchan prior to the Physical/Special split in Gen IV, since they all worked off the Special Attack stat, which Hitmonchan's Sp.Atk was a pathetic base 35!
Similarly, neither of Gengar's STAB moves were of much use prior to the split since both Ghost- and Poison-type moves ran off the Attack stat, which Gengar's is a pretty miserable 65, which is only half of its Special Attack of 130.
Sword/Shield introduced Body Press, a Fighting-type move that uses the user's Defense stat to deal damage instead of its Attack. While most Pokémon that can learn it have a high Defense stat, Wailord's Defense is so crap (45) that it is not worth teaching that move to it. Its pre-evolution, Wailmer, is even worse; with 25 Defense, Body Press will barely leave a scratch.
The weight-based move Heavy Slam increases in power the heavier the user is compared to its target. Flapple also gets this move, but as the lightest Pokémon to learn the move (2.2lb/1kg), but it can only get maximum power from the move against 7 Pokémon; that is less than 1% of the entire Pokédex!
Going back to Salazzle, she is easily one of the faster Pokémon to come out of Sun & Moon with her 117 base speed, and combined with a Sp.Atk of 111, she is primed as a special sweeper. However, Salazzle's special movepool is extremely shallow, with most of her special moves confined to her Poison/Fire STAB combination, giving Salazzle very limited options as far as coverage is concerned. She does learn a few strong Physical moves, but they aren't all that useful running off of a subpar Attack stat of 64. As previously mentioned, she can't even stall effectively because of her weak defenses.

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