As I understand it, Michel's rationale for requiring a request for invitation is to experiment with a different kind of community, where people know each other. This may work in this case, where Michel is very active in creating and soliciting in-depth content.
My inclination is still very strongly towards openness, though. I have seen very constructive anonymous edits to Appropedia immediately after I've blogged on a certain topic - that's much less likely to happen on a less open wiki. They might leave a blog comment instead, but the direct engagement gives more power to the contributor, is more satisfying, and is less work for me.