[I use Dropbox on a Mac; this (problem) might not apply to other operating systems.]
I would like to Backup, for example, my Documents folder on multiple user accounts using distinct targets on Dropbox. The current implementation does not seem to allow this. I think the problem stems from having tied the target backup folder name to the computer name. It seems to me that allowing the target to be specified by the user while setting up Backup on each local OS account (user) would make this feature MUCH more useful. Maybe an "Advanced..." button on the Backup set-up dialog would be a good place to allow the user to specify this.