Yes, you can recover deleted files in Dropbox but to do that, you must know you deleted something, and then, what you deleted, or you must periodically check for deletions in the Dropbox UI.
Also, Dropbox probably will happily sync files that were written with bit errors on your hard disk.
Acceptable risk? I think so, but it means accepting some decent risk of losing photos.
My point is that many people likely would be happier if they spent, say, two evenings a year watching past photos together with their loved ones than when they spend that time making (more) sure their collection stays perfect (and at thousands of photos a year, two evenings is _nothing_ if you want to make sure your archive is bit-for-bit safely stored and properly indexed so that it is reasonably easy to find photos on demand)
