(Op here)

Basically what others have said - I want things Synced, not just backed up. I like being able to access all my things on iPhone, on other computers, etc, and with it always working.

Also, I used Dropbox first, Dropbox "opened this market" for me, so to speak, which means I wasn't even thinking that this kind of thing would be convenient before I installed Dropbox and started using it. It's very possible that there are better services which I don't bother looking at because I'm with Dropbox.