If I’m reading you correctly, you are either thinking that the system would FIND FOR YOU 2-2-2 and match them together or you would QUEUE AS one of the 3 types.
If it’s the first, you could get (as an example) a 3000 level Tank that’s trying to learn DPS. Since they didn’t have to specify, the system just read that most of their hours are in tank, that’s really not their fault. I don’t think anyone really wants this system where the MM assigns you a role.
In the second instance, the real concern is that the same people that refuse to swap off the 4th DPS will queue as support to save time and either a) continue to play a 4th DPS or b) play support very poorly.
Whether it’s a, b, or both depends on how they implement Role Queue, but that’s the concern.