The argument is that without rigid identity on the web, anonymity and pen names could not really take off and receive audience. Many individuals' first encounter with the Internet and any web communities is with Facebook, and this is because these people would not feel comfortable starting out on a place like Reddit for 4chan. For this reason, rigid identity is a necessary foundation for the usefulness of pen names and anonymity. Moot's argument that pen names are the way to go is flawed insofar as it undermines the "necessary evil" of rigid identity, which is a natural, socially evolved stepping stone for anonymity's value.
Without people going online and possessing monuments to themselves (facebook profiles) in a way they feel comfortable and secure in, anything posted online (including content under pen names) is in aggregate less valuable. Rigid identity is a necessary characteristic of social media content which can result in inexperienced web users taking the plunge to explore the rest of the less-rigid web.