The OP considers versioning files but does not address collaborative sharing, branching, merging, etc at all. Dropbox and the other alternatives suggested in the post do not simply offer anything for merging or dealing with conflicts (or do they? what about versioning filesystems?).
When working alone on a fairly straightforward, linear project, you might actually get away using Dropbox. Try to work in a team of any substantial size and there will be trouble.
Having used Dropbox for a couple of random personal projects a few years back I can honestly tell you it's not a good idea. It's better than nothing but Github is free, there's absolutely no good reason not to be using source control.
It's simple things like not having more than a month's changes or having commits you can roll back as a group.
When working alone on a fairly straightforward, linear project, you might actually get away using Dropbox. Try to work in a team of any substantial size and there will be trouble.