Nothing. That actually is a method that's been used by "evil doers" in the past. The government had been monitoring email correspondence using Carnivore. What they found was that instead of sending emails back and forth, a shared inbox was being used, and information was simply being communicated using progressive edits of email drafts. It lived on a server, so it was available for discovery ... but since nothing was flying back and forth, it took awhile before it was detected.

Probably nothing, but it could trigger some anti-hacking heuristic that would disable the account.

It's also much less convenient than just sharing the link.

Plus some troll would just delete all the shared files.

What equally stops someone from simply logging in using those account details, deleting all the files, or simply changing the password? Feels like whack-a-mole to me.