There are many sync agents out there that work already but it would make a nice openntf project. It would have to let each user select what they want to sync to. Or how about federated address book, like the calendar.

I have done sync agents but always for a CRM app since the mobile directory took care of the domino directory.

Maybe for small shops...when larger firms have 200+MB (being kind) Directories for thousands of users, pulling in each respective user's contacts and recent contacts would kill server performance starting with view indexing, while ballooning that NSF's size and then having to replicate all of that data across the enterprise, only makes additional work for admins to write selective replication formulas to avoid a huge pipe (bandwidth) killer at small remote offices.

The "Copy into Personal Address Book" action is loathed by many admins, as it definitely creates routing issues when that user's name/address is changed--for starters.

Nice in theory, but the implementation seems, well, like there are better ways to do what you might like...if you want to backup the data, user Domino roaming user profiles.