Msq shinanigans aside, you could have 9 tanks and healers and 3 or 4 dps, and most players would still play dps. It's a pretty solid fact that most people don't want the stress/complications of playing a tank or healer. Especially when they tend to get blamed for any and all problems in party situations. So most play dps to just fire away at enemy's.

Yes, another healer and tank or 2 *may* help a bit. But more than likely it'll be current tanks/healers testing out the new tanks/healers.

Also, as a few others have said, I'm a healer. I play sch. But I'm currently occupied getting Sam up as it's an amazing class I've been waiting for. And I probably won't touch my healer much outside of friends asking for help with lower stuff that Ill run