If you notice, each generation Ash really just uses a small cluster of Pokemon, whereas all the other Pokemon he captures are usually just back-up Pokemon for the big matches after every 150 episodes or so.
These Pokemon usually get minimal screentime while Ash is actually traveling:
Kingler
Muk
Tauros
Snorlax
Heracross
Donphan
Torkoal
Glalie
Gliscor
The only core group teams Ash really had:
- Bulbasaur/Squirtle/Charizard....around for 150 episodes, Kanto, Orange and early Johto, and reunited at the end of each generation.
- Bayleef/Cyndaquil...the only two Johto Pokemon that got good screentime, were around for about 100 episodes.
- Sceptile/Swellow/Corphish...on Ash's roster for nearly 180 episodes, from the beginning of Hoenn till the end of BF.
- Grotle/Monferno/Staraptor/Buizel...on Ash's roster for almost all of DP, so far about 130 episodes.
Why do you think Ash really only uses a small group of 3 or 4 Pokemon each saga, whereas the rest of his captures either get less screentime than the others, or are Oak'd immediately?
Kingler: I agree with Kingler. It was never given a personality, the most interesting thing being the revalation that it was half the size of Gary's while it was a Krabby. It did get used in the Kanto Cup, a lot (Cloyser, and the 1st round battle it did all on it's own), and was also used in Johto to some degree, truly showing it's power in the Whirl Cup (Splitting waves with Crabhammer) I feel like they pulled out when Kingler was going to be used in a leauge, then, a group of Voltorb fell on it, and knocked it out.
Muk: Was used for comic relif in Kanto, and Johto, until Bayleef took over it's role for a short time the role being causing Oak pain. Muk has been shown to be powerful, however, Janet's Bellsprout, which beat both Pikachu and Bulbasaur was effortlessly beaten by Muk.
Tauros: There are several reasons for Tauros' lack of personaility. Firstly, there are 30 of them, so odds are Oak dosen't get the same one all the time. Secondly, it's a powerhouse used only for Leauges, and Battle Fronteirs. Thirdly, the ep where it was caught was never aired away from Japan, and that could have some effect on the minimisation of Tauros' role, most outside of Japan don't know how Ash got it.
Snorlax: I disagree, Snorlax was developed quite well in the OI saga. The Fronteir saga also made it shine once again.
Heracross: Was forgotten towards the end of Johto, sadly. Although, it's battle against that Scizor had to be one of the highlights of the whole region, especially seeing as it wasn't a Gym or Leauge battle.
Donphan: Be reasonable. Ash got it late in Johto, and it was used for the whole of the BF saga, Donphan WAS given development. It went from a baby Panphy to the powerhouse it is now.
Torkoal: Got a lot of screentime. Used for comedy, but was also massivly powerful. Not used in Gyms because Hoenn's didn't support the Fire-type well, and Torkoal dosen't suit Ash's speed-based style. Although, it did get good time in the Leauge, and against Registeel, putting up a fight.
Galie: See Panphy. Gotten VERY late in the show, yet was given development, from a Rascal Snortunt, that couldn't control it's Ice Beam, to argulably Ash's strongest Hoenn pokemon. I for one don't see the power increase as exaggerated, whenever Ash's pokemon evolve, they get a large power boost, and mastering Ice Beam, one of the most powerful Ice type attacks, is a leap in power in itself. It'll probobly come back for the next filler too.
Gliscor: No, he's part of the Core. If anything, it's Pikachu who's been relegated to being little but a mascot now.