See: http://blogs.testbit.eu/timj/2005/06/30/28062005/ (page moved)

Wiki access. When i come across a minor error or easy-to-fill spot on wiki sites like Wikipedia, i often take the time to edit the content appropriately. I can’t be bothered to create an account for such minor edits though. Especially not if i’m unlikely to ever return to the webpage since creating an account means generating a new password (sometimes even an untaken username), and taking notes about the site and the newly generated account data.
So wikis meant to absorb contributions from occasional visitors need to accept anonymous edits, even if that means regular anti-spam edits have to be carried out.
Consequently, beast has such an open wiki. And yes, i’m deleting spam including history from it and keep a spam word filter updated on a daily basis, because i do think open access is worth it.