i do think it would make things a little more interesting to have a Support role, and a Control role in the mix...
Tanks could focus on taking the big hits, while Control help direct the enemies away from the squishy targets and onto the tanks while providing better dps than a tank would, but less than a dps would.
Supports provide healing and damage reduction, not as much as a healer, but help provide a baseline of incoming heals to lessen the spikes of incoming damage. (like shadowpriests of old).
While I can see progression heroic guilds tossing out the support & control roles in favor of pure dps/heal/tanks, I think more casual guilds would *love* to have a control in the group to be able to step in if a tank goes down and needs a brez, or gets otherwise incapacitated, etc. would give blizzard a little more flexibility to make fights a little more demanding on occasion without changing the amount of actual tanks needed.
Same with Supports, makes it a little less stressful on healers that are just dipping their toes in the water, but also lets a 'pro' support keep people alive, while dishing out a good deal of damage.
and yeah, it would speed up queues significantly.