That is never really backed up, and doesn't make a lot of sense.
I've been doing this for a while - but using Dropbox as my working directory [1]. All he is doing here is using Dropbox as a backup - but such services exist (e.g. BitBucket) and are easier to sync to (one push and you don't have to wait for Dropbox to then sync).
Dropbox is useful for maintaining a consistent working directory between computers. I can hack around at work, go home to finish up and then commit. Brilliant :)
1. http://www.errant.me.uk/blog/2010/04/a-nifty-way-to-manage-c...