@rawker: While that is true, it would still be a better situation than if players could straight out prove that one class is considerably better then the rest. Bias will always be there no matter what you do... unless you just have one tankingclass.

@Brian_: As long as all tankingclasses can still survive the encounter without being a liability for the group, the one with the highest DPS will be prefered by the majority. What you're talking about is pretty much just turning the problem upsidedown instead. Making the "tankier" class be the prefered choice because the more "dpsy" one will have problems surviving or just being a liability to the group. It's not really something that would make the classes balanced if you think about it. Real balance is when you can't point at a given situation and right out say that class A is better for this, as class B can't handle it as well.

Also "theoretically possible" doesn't mean that it's realistic. This is an MMO, and the vast majority of players will always pick the "best" classes when given the option, which in most cases means the classes that can get them through the encounter as quickly as possible. I have yet to see a single MMO that managed to have a class setup like this where all tankingclasses were on equal footing in the eyes of the playerbase.