The only downside to that is what to expect from people when you group. You queue up to do an instance (or w/e direction they go), and join a group with Johnny Derp. Johnny Derp is your Tank, but everything he has is tuned towards being a DPS. In fact, you as a Healer, have more HP than this supposed Tank.
If Im playing a DPS and have a 20-30 minute queue, this is the last thing I want happening. If I leave the group right away, the abandon debuff is applied for 15 minutes and then you have to wait an additional 20 minutes to join another group. An hour wasted, just because Johnny Derp. God forbid if you join another group and that tank, or healer, present you with the same problem.
Ie. Classes with specific abilities, tuned to specific gear is much more friendly. If every player was as experienced as the average forum goer - then this wouldnt be a problem. But its not like that, and you have to think of the worst shit you might have to deal with when theory crafting.