Just looking back through my track record, like 85% of my "success", so to speak, has been with heels. I've won world titles with six different characters and only one was a face, which was my first character ever used. I'm one of those guys that just prefers to play as a heel over face, and really it's because they're just more exciting to play. As long as you stay in character, you can just abuse the fuck out of an opponent. It's more entertaining than saying "You're great, I respect you" etc.
The way I see it, faces tend to always back up faces, but heels not so much. Heels don't give a shit and care little about making friends, but faces it's different. Seeing how the majority play heels, obviously it's not good for the balance, but you can pit them together in matches no problem.