The main one, for me, is that it anchors the mix if done properly - phantom centre can sort of twist or dislocate the soundfield if you listen carefully, losing the main anchor point - and a dedicated centre channel gets around this as well as leaves you with much more flexibility & room.
Some things are best in phantom, but not all - and I honestly believe (note the word "believe" as this is just an opinion of mine) that any 5.1 mixer who avoids the centre channel just does not know how to use it properly.....