I have found one case where it makes sense. Say you have a modal popup with an 'editor' widget in it. If that widget then needs to popup another modal for adding a link, it makes sense. Here is a screenshot of what I'm talking about...
http://www.evernote.com/shard/s178/sh/003f6b5e-0869-44af-ab9...
This is also using the bootstrap modal solution referenced in this article. Redactor is the editor widget.