Running the Twitter Fan Wiki (
twitter.pbworks.com) I've found that the registration barrier is insufficient. Spammers were still able to get through and trash various pages or upload nefarious content and whatnot. I had to move to an ask-for-edit permission policy — and only approve people if they can give me a hint as to what they want to edit.
Now, it's probably a result of Twitter's meteoric rise in popularity that I've seen this kind of abuse, but it's still smart to be careful about being too generous with permissions — especially if we don't expect the pool of editors/contributors to be all that wide.
But, if there are many people watching for abuse, I'll defer to others' judgment.
Chris