It's great for packaging offline patches, portability, and in many ways more flexible than updating through Windows Update. What does Firefox access on WU have to do with that?? That's not nearly the reason I believe most are using AutoPatcher, and I think Microsoft is missing the problem here, or more likely, they either pretend or don't want to realize the problem. I mean, all Windows users have IE installed anyway, including Firefox users.
AP is more for system administrators and experienced users wanting to package files for easy offline installs, possibly on a network scale.
And of course they'll deny it's for WGA reasons. They don't want to get into the debate of locking in users to their services out of inefficient antipiracy methods again. They'd probably even lie to not have to.