It's not worth playing the game for the experience of the game itself; it's one of the worst RPGs around, and I'm only comparing this with turn-based RPGs. The idea of going around earning 8 badges, becoming the Champion and completing the stupid Dex sucked in the first place. People get the game because there's a new generation of Pokemon, and new strategies for competitive battling. That's why. As for those who don't competitive battle, they get it because they love their Pokeymanz.
Well, I don't think collecting the pokedex is lame. It takes a lot of work, and it's by far one of the hardest things to do in the game. I do think they should change the whole gym thing by now, but collecting the pokedex makes the game really fun, and most people do not complete it.