I use PO simply because my WiFi absolutely refuses to work with my DS. That and the fact that with replay viewers now, I can't make the argument of preferring to watch the moves happen
and Like all those before me have said, PO is instantaneous. No training, no breeding required. MUCH easier to build teams that way than to breed a Mon, train it, then find out to don't like it or that you need/want a different nature.
Oh yeah, and the fact that PO battles have a faster pace anyways. DS battles take too long for me. When I battle I like to finish it as quick as possible.