An amazing healer can do great things for thier team but an amazing dps will carry a team to victory more often than a great healer will. As a healer all you can do is keep others alive, if they can't do anything then you're pretty screwed. As a good DpS you can control the entire match.
I'd rather have 5 good DpS on my team than 5 good healers. AS others have pointed out, thats just boring for starters.