What job roles a class branches into doesn't particularly matter because it's the class that levels, it takes 2 seconds to swap from one job to the other. So long as they occupy different niches (not necessarily different roles) then I don't see an issue with Rogue branching into Ninja and Thief jobs.

I'd love a Rikku style Thief job working as a pseudo-support melee-bard type job, but whether or not that will work with Rogue/Ninja really does remain to be seen.

Edit: Because Arcanist branches into a DPS and Healer people seem to jump to the conclusion that it has to be that way with every class but it really doesn't, it would be nice if most classes could end up that way as it would really help queue times I think, but SE shouldn't try to force jobs that won't work onto classes just to try and make sure every class can fill 2 different roles.