The reliably thoughtful Chris Messina just posted on the benefits and costs of OpenID, partly in response to a decision by linkmarking site http://Ma.gnolia.com to stop accepting traditional address and password registrations in favor of OpenID only. http://FactoryJoe.com/blog/2008/03/30/magnolia-moves-to-open...

In the cat and mouse game between site owners and spammers, data on recent Ma.gnolia registrations showed an overwhelming number of spammers have begun using automated tools to proxy Captcha and create traditional address and password logins. For now, OpenID encumbers them by requiring presumably more valuable credentials.