Why would I want to be forced to use South? There are other migration methods that approach the problem different ways, or perhaps I want to write my own?
Third part != Lower quality. In this case, it means being able to make incremental improvements outside of the Django release schedule. So while Rails bakes it in and forces a release schedule on migrations, South isn't encumbered by this, and is more agile.
But I'm using Django DESPITE the fact that I have to use south. Contrast this with Rails, which FORCES you to use migrations (last I checked).