i doubt changing/adding specs would have bigger impact than adding new tank, people play what they enjoy, most people who enjoy doing dmg as mage wont suddenly love tanking as "battle mage" (or whatever new spec) even if they add it, theyll just keep playing mage dps...
and if they outright change the spec to tanking theyll simply play other specs or change class if they dont enjoy any of the other dps specs
sure some people would start tanking if their class get the chance, and some people change mains, even if there is no new class, so i doubt the impact would be significant in either case...
tbh, i had idea how to basicaly reduce the number of specs while making more options (in short, 2 specs per class, 2 "subspecs" per spec) which would make possible all classes to have either tank and spec, bcs i think the more options the better, but i still doubt it would have significant impact