Would love to see good KBM support on the PS4 version but hard to see how it will go down smoothly for pretty much the reasons Obsiv said.
A lot of controller users, especially the ones with competitive attitudes will not like them in the same matches as them, so they'd have to add filters for Controller only, KBM only, and mixed.
If the filter route was the way they went, then they'd probably have to set Controller only as the <default> and let you choose from the other 2 if you wanted. Then it becomes a question of are there enough KBM PS4 players to keep the servers nice and packed without always seeing the same people over and over.
Then theres the route of allowing PS4 KBM users to join pc servers instead of going against other console players... but considering how PC versions always have way more server options, not sure how easily they can do that.