As an additional example... a lot of us have someone else check / manage our calendar. It might be coworkers trying to schedule a meeting and they might rely on your calendar being up to date so that they can schedule themselves into it (this is a pretty common practice at large companies and some smaller ones). Alternatively, you might have someone like an admin or assistant scheduling you into meetings, patients, etc, and they need your latest availability to do that effectively.
And for that matter, when I'm in the office, the computer screen (my iPhone syncs with the Apple apps on my home computer and not Outlook on my work PC) would be much more convenient for seeing my calendar than the iPhone screen.
I don't have MobileMe (yet... right now a big killer is this issue with all or nothing support of Exchange calendars -- that hasn't been resolved, has it?)... but those are some reasons it would be useful for me. Aside from my calendar, once a week is plenty often to sync my iPhone.
Agree with TBi... MobileMe I think would be fairly silly if you don't do any business with your mobile device. Although, sometimes if you're a family and you're trying to schedule together time with your honey... well, that can be tough also. Hell, that's tough for me just dating and there not being any kids involved. 😱