Yes, I'm aware of that. This helps, up to the point where you don't notice that something's gone for 'a few days'. Let's say you have your student project's folder shared with two other colleagues. You take a few days off, use your PC for casual browsing, meanwhile your colleagues are working on the project. At the end, one of them (who doesn't quite understand how dropbox works) deletes the files while the other is still working on it and dropbox gets confused. Your PC gets synched at a moment where you don't notice. Come back from your holidays and you will have a nice surprise waiting.
It doesn't matter whether he notices it or not, you can't count on social factors when it comes to backup systems. He might notice it but then he thinks, oh no problem, it's all in Dropbox anyway, I'll talk to Dylan16807 when he gets back.
I've never had any complete data losses until now, but I've had this situation where the Dropbox cache was the only place to retrieve files twice and it showed me that it can't be trusted this way. I'm certainly not gonna wait until the real deal happens just to have a personal story on how Dropbox can go horribly wrong. I've been shown the possibility and that's enough for me.
As for specifics, the case I mentioned is the one I saw where it can go wrong. IMO the only really save way to use Dropbox without additional backups is if you never share folders and only ever use one system with write-access at the same time - which is not the usual use-case of that product.
I just want to understand better why you had to resort to the cache. The only time I've ever done that was when I accidentally deleted a bunch of data and didn't want to restore each file by hand or put in a support ticket.
I can't quite put my foot down on what the scenario is exactly. One time might have been a case where a folder with lots of small files got moved away and Dropbox only allows single file restore, which would have taken hours, possibly longer than the lost work. Once I've done something stupid where I wanted to exchange a whole Dropbox folder with a different version. I switched off the client on one PC (by mistake, obviously) and exchanged the folder on the other, then switched it on again. Then I became aware the the old folder had important stuff in it. The versioning was corrupted at that point, many old files would either not show up online or would produce an error message.
So usually there is some user behaviour involved, yes. But the whole point of a backup system is that you can rely on it, even when the user behaves stupidly up to a certain degree. Sync is for day to day collaboration and data management, it's not for backup.
If dropbox works well for you for source control, that is great. But frankly, if you get to the point where you have to start writing hacks to keep it working, it is probably time to move onto something designed for the task.
