One of the big drawbacks for earlier versions of Direct3D (9 and earlier) on PC was that draw calls were expensive operations, especially compared to OpenGL, because they required a trip to kernel land and back again. By moving the application into kernel mode many of these calls become much less expensive.

The obvious down-side is that if something goes wrong the whole system is hosed. This isn't really a problem though because there are no shoddy drivers written by half-caring vendors on a console, content for the platform had to be certified to be released for it and was required to not crash, and even if it does crash it's much less aggravating because, hey, it's a video game and not your unsaved work that is lost.

You are only ever running one process at a time and it runs under a hypervisor anyways.