Pro.
Better matchmaking based on your best gear and skills on particular character.
Ability to control the amount of classes in one team.

Cons.
Flexibility is gone.
Player base is not big enough and matchmaking will takes longer for sure.

But flexibility is not really an issue because usually (on PC) lobby's is gone after a one or two matches because of... "oh my god i will not play against a shaman", or .. "they to powerfull i will loking for someone else..."