You'll still be waiting till the tank ques.

All I play is Tank, I still have to wait for another tank if its an 8 man party. I grab a tank friend, the que is instant. Doesn't do shit if you increase the cap to 5 dps, still waiting on tanks.
Instead of throwing in an extra player, give that extra player reason to consider playing Tank/Healer.

Clearly even with the copious amounts of incentive to play these roles (fast/instant ques, mounts, exp/gold bonuses) its not enough for some people. Probably poor marketing on how fun tanking actually is.

Because if that 3rd dps just played tank instead, 3 other people would get a run.