So github can be a part of a backup strategy but it isn't a strategy by itself.
Likewise, there are plenty of parties that wouldn't dream of storing their data in a third party repository, it could be compromised, there are at least 'n' github employees that now have access to your data etc.
So there is a need for both options, one where you outsource your headache to github and keep a couple of local copies just in case, another where you do have your own repository that you control with the associated backup mechanisms and a number of off-site copies.
Fortunately github makes it easy to do the former and git itself can make it (relatively) easy to do the latter.
For plenty of people the first is enough. For me it wouldn't work, so I'm really happy this got posted.
It runs in cron and backs up all my data daily.