Allow for people that choose support to not have to choose another role

Let's see, this would help things out. Right now, some support players still end up having to play other roles some times. This is bad, since the number of games the matchmaking can put together is essentially almost entirely gated by how many support players it can find. So it actually makes it take longer to get into matches when a support player ends up playing another role, since some other game won't be able to use that support player. And there are definitely not enough dedicated support players to the point where they'd be common enough for the people that only select support to ever cause long queue times, since there are seemingly always way more people that want the other roles.

Oh and here's the clincher for you. The disadvantage? There is none, except maybe people complaining that there should be for other roles, but that would really increase queue times so it's not worth it.