Again, as best-illustrated by Apple’s official iPhone / Touch applications – iTunes Remote, Keynote Remote, iDisk, and now, Gallery – there’s a definite, purposeful limitation to the tasks the applications perform.
Each MobileMe-specific application in particular, only focuses on one aspect of the MobileMe service: iDisk and Galleries.
Apple could have easily released an all-in-one MobileMe app, but they didn’t. They could also just as easily bundle the free apps with the device, but again, to maintain simplicity, they’ve – so far – chosen not to. It’s up to the user to install (enable) them as necessary.
These are, of course, good design decisions. But I think they’re much more than that too.