It's true that git+Dropbox isn't resilient in the face of multiple simultaneous writes, but I think the problem is overstated. You might see an inconsistent state, but you'll never get corrupted data because of it. You might corrupt the repository on Dropbox (has happened to me a couple of times), but this is trivially fixed by deleting and recreating it.

I'd never use it for a large team, but for a couple of collaborators, or syncing a repository between multiple computers for a single developer, it's a nice and easy solution.