They've already said that the split will really only matter if you're looking at Savage and Ultimate level content, and even then it might just be a case of it being easier or more optimal to go with the intended split. All tanks will need to be able to perform the tole at lower difficulty combat content anyway since in 4 man content you can only have 1 tank or in Alliance Raids you need to have 3 tanks between the 24 players.

Basically we just don't know enough yet because we haven't seen what the toolkit of an Off Tank will be compared to what they have in mind for a Main Tank, and we also don't know how fight design and mechanics are intended to change in order to emphasize the split. Hopefully they will come out and clarify this soon so that we don't have to wait until Berlin (or even after) given the amount of hand wringing and speculation over the issue.