Hmmm, I've never had much trouble rolling back to an earlier version with Dropbox. Being able to add comments and see differences definitely sounds nice, though.
My main problem with it is actually finding the right version to roll back. It's easy when you want to roll back to version of one file from 5 minutes, but if you ctrl+s all the time like me, getting few files to version from few days will be pain in ass, cause (unless it was changed) Dropbox lacks a good interface for checking out versions.
Also with DVCS you can easily clone, merge and branch your code, which is not something you do often as a solo developer, but if the time comes, it makes everything easy. AND it let's you integrate with bug tracker/to-do lists and all that stuff, which was the main feature discussed here.