And how do you imagine that it would happen that Dropbox would randomly trash the contents of your files? And if it was that buggy why the bug be limited to just the files in your ~/Dropbox/? By that argument you might as well say that it could trash any file owned by your user. And if you believe Dropbox to be likely to have such serious bugs, why just Dropbox and not other software you are using? For example Chromium or Firefox? Or any other piece of software?
Seems less like a serious concern and more like trying to think up any imaginable thing that could conceivably go wrong no matter how unlikely.
Lots of ways. Suppose they tried to implement a 3-way merge system (a la git) to resolve conflicts in your synced files. It's easy to see that going terribly wrong if they implement it incorrectly.
It's proprietary software as a service. That means they could conceivably make a breaking change like that without even notifying you, the user, and cause data loss.