Imagine a world where GPLv2 was found to be unenforceable in a major jurisdiction such as the US. It has been a serious worry in the past. A CLA lets a project survive that situation. But I don't know if it is possible to have a CLA that also cannot be used to fork a project into a commercial license. So the trick would seem to be a CLA that assigns copyright to some sort of non-profit that is legally bound to be unable to take that path.