Well, just as there is big diversity in pokémon that do evolve (from some generic Watchog with 2 stages, to pseudo-legendaries like Salamence), there is quite different non-evolving pokémon.
-gimmicks
There is plenty of those that are mostly in existence to show off some game mechanic, regardless of how useful it is in the competitive sense
This includes Ditto, Unown, Smeargle, Spinda, Castform, Kecleon, Chatot
-weak
Then there is those that are as weak as any regular pokémon that is yet to evolve...they just never do. Those are the ones most people put in the same category of "waste" as baby pokemon. But we know from examples like Aipom, Murkrow, Nosepass (and lets not forget Mega Mawile!) that not all hope is lost.
Including Corsola, Qwilfish, Pachirisu, Farfetch'd....and the winner is: Luvdisc
-strong
Ranging from 2nd-stage-ish to 3rd-stage-ish power, these seem like the opposite. Pokémon that are encountered like they skipped the weaker stages and start off "already fully evolved" (and mostly later in the game obviously).
But as any other, these can also gain any number of stages later, as the prime examples I would give for this category would be the classic dozen fellas coming before Magikarp, like Magmar, Scyther, Mr.Mime..
Still valid inclusions are Tauros, Skarmory, Shuckle, Tropius, Druddigon, Cryogonal, Hawlucha, Lapras etc.
Anyhow, for the weak ones I always hope for new evolutions, because I'd love to use some competitively, which just doesn't work when they're as weak as that. As for the strong ones, although I do favor ones that evolve for some reason, but I try to use these every now and then at least for the heck of going against my bias towards evolving ones.
Unlike many people, I'd love to see baby stages added to any number of these, like Tropius, Lapras and Skarmory (rather than already weak ones with evolutions, like Pikachu and Clefairy, getting them), because just like Elekid, Tyrogue and co., they would add some spice to them and probably make them encounterable more early in some region etc.
And for the gimmicks, I don't think they should be left untouched simply because they are gimmicks.. in most cases, their gimmick could be exploited and used to make them the best in whatever they do. For example, I would make Castform evolve, but differ depending on what weather it evolves in, losing its Forecast ability for the fitting weather ability (Drought, Drizzle, Snow Warning, Sand Stream), making it THE prime weather-inducer, with all the others (Politoed, Ninetales) being alternatives rather than the rule.
