With a DVCS, you don't need commit privileges to benefit from all the advantages of using a VCS in the first place. So even if you keep the exact same list of comiters, "second-class" contributors can develop things much more easily.
It also means that you can "screw-up" until you push (and as such can put things for review, etc... more easily).
It's a bit more convenient, but svn plus private repositories, or SVK, or git-svn work equally well.
Again, I'm not saying git isn't more convenient - it is; the point is that even git-using projects end up with a "main repository" to which only a small number of people can commit. Increasing the number of people with commit rights can be useful even when using git.
Also with a model similar to Github where you fork the repository and submit pull requests, Python developers could cherrypick the best commits from anyone who is interested in contributing.
This probably doesn't scale, but you could certainly do it for just the predetermined contributors.
It also means that you can "screw-up" until you push (and as such can put things for review, etc... more easily).