I have mixed feelings about sites that rely upon the "we regularly purge identifying information" excuse. In this case I believe it makes the site directly responsible for libel as they are actively looking to protect the sources of information that benefits them.
This isn't about someone defending a champion of free speech or a hero of mankind. It's about, in most cases, liars who invent claims in comments to support their side. Such lies gain truth through repeated assertion (remember how RIM couldn't believe that the iPhone was possible? Retold endlessly, it was all based upon a quickly deleted claim on some random message board by some anonymous person claiming to be a RIM employee. The claim was absurd -- the iPhone used the same stock ingredients that RIM was used to -- but there you had an "insider" to support it).