One problem with the roles is they change based off of who is in the group at the moment.
If lets say me, gomo and Alan are the group for a moment Alan would be the tank with me as dps and Gomo as debuff and cc. You add Random in that same group he would be the tank and Alan would move to DPS since he is more of range dps. on contras if it was just me and Gomo, I would have the tank (I think my 12 ac is higher then his) with Gomo being the DPS and debuff role. This is not saying this group would but ....
Our group keeps changing alot since people have had to change toons, people come late, or have to leave early. So this is what couses the problem with that.