I'll regurgitate what's already been said. I hate 'em because not only does the realism of having to reload go right out the window, but it also changes the tactics. And by that, I mean they just disappear. Nobody aims anymore, nobody has to take their ammo count into consideration...Airsoft basically becomes Paintball with nicer looking gear.
I don't "hate them" hate them...I'm the only guy in our group who doesn't use them, so it'd be one hell of a lonely game if I refused to play with those who do use high-caps. I just won't use them myself.