I'm a cross between an EV trainer and a "just for fun". Each pokemon I use in battle is EV trained to highlight what I believe are their greatest strengths (and polish off their weakest attributes), and I enjoy a battle that challenges my team. However, I won't write one off my team just because it's in a lower tier. Actually, taking pokemon from separate tiers and placing them in one team shows just how diverse one is as a trainer, if anything. I dislike using ubers though; if I won, I really wouldn't feel like a victory if I used my cheapest moves to decimate the other team.
At the end of the match, though, it's all about fun... and if you don't play a game to have fun, then you've probably lost sight of what a "game" really is.