There seems to be a lot of big generalizations about why people play Pokemon. Most people I know don't try to complete the dex anymore. "Catch 'em all" hasn't been Pokemon's tagline for a long while, and I consider the huge list of Pokemon more valuable for keeping the game from becoming stale than they are for merely collecting.
I am not quite what I'd call a "competitive" Pokemon player since I rarely play against human beings. I mostly play against the computer in the battle tower/subway, sometimes solo, sometimes coop, and therefore enjoy raising different Pokemon to try new strategies. A huge list of Pokemon means there's loads of combinations of types, stats, movesets, and abilities and it's impossible to have a perfect team. I dislike battling being overly centralized around a few "best Pokemon," or the idea of a Pokemon only having one counter. Every time they release new Pokemon, the game gets more fun by giving me more choices and strategies to try out. They improve the experience whether I'm catching them all or not. It also makes the first play of a new generation Pokemon game loads of fun, thanks to the experience of discovering and learning about the new additions through battling and raising them.
On the topic of being a 100 Percenter, I agree that some people take it to extremes and do thoroughly unfun things in the interest of completion. I also think that sometimes, a game is fun and it's great to have more excuses to play it. My leisure time is valuable, so whenever I bother completing anything optional in a game, it's because I'm enjoying myself and want a reason to play more. I think Pokemon is fun, so I do fill the dex. If it becomes a grind, I do something else for a while. In Black and White, I've been alternating between levelling up Pokemon for evolutions, hatching eggs, raising Pokemon for the subway, passing up Pokemon from Soul Silver, doing daily events, using the GTS, and fighting on the Subway. There's plenty of different things to do that keep me having fun and make it so I don't have a dull grind.
In short: there's more than one way to play Pokemon.