So you can need on what you want, cool, fine. And perhaps people will roll tank/healer more that way. Again cool and fine.

But not only will it make no difference (the amount of DPS compared to tank/heal is just crazy) it'll create another problem because people will rage (and rightfully so) that classes take loot they SHOULDN'T easily be able to get with a Need roll. There's a reason it's there, you know.