Don't get me wrong, I've trained many non-evolving Pokemon, too (Absol and Zangoose being among my faves). But I've ranted about my dislike for one-stagers in another topic... In short, there's less to work towards, just brain-numbing leveling up and tweaking their movesets to your pleasure. It's just not as great an experience as raising something that evolves, in my mind at least.
Let's not even mention the crappy one-stage Pokemon... >>