Bottom line: registered users make the vast majority of edits (and, as of 2005, the top 1400 make 73% of them) but it's IP editors (and small-time registered users) who contribute most of the content.
These (and others) are all good points. I've been also wondering if banning anons would be throwing the baby with the bathwater.
However, from my own experience of watching 800 articles daily, most anon edits are bad, and the time I spend now reverting vandalism could actually be used writing things. This is just a sample case of course.
Anyway, I am very sure at this point that the Citizendium people are making a mistake narrowing so much their pool of editors and with extra regulations. Whether that means that the Wikipedia model does not need more editing restrictions will remain to be seen.
Oleg Alexandrov