Most notably to implement some kind of better library sharing across multiple stations. It'd be great, for example, to have my aperture libraries centralised on my main NAS drives and accessible by Aperture 4 from a laptop, desktop, 2nd laptop when necessary. Syncing libraries this way seems to be the way to go, with each computer happily coping with it's own prefs and UI changes etc. without disturbing the master file system.
Selective sync would be the way to go here. For example, I'd love to have one big Aperture Library and sync it between devices but raw Library size limits me in this respect. There is no way I'd be able to sync an entire library to my MBA for example. Being able to sync Library layouts and then selectively sync from there on a Project/Album/Folder basis (nothing, thumbnails or the whole works) would be ideal.
The biggest issue with a sync situation, of course, becomes dealing with conflicts. Integrate a good version control solution into the Aperture Library though and you'd have that taken care of. As a bonus, you'd get revision history and a great deal of other features out of the deal as well.