I've written about anonymous and pseudononymous writing before. I think both forms are valuable in certain contexts, but I think it's unfair for someone who won't write under their real name to directly denigrate another person. It's fine to attack policies and positions, but using an assumed name to make a personal attack -- even if warranted -- just doesn't feel right to me.
In some circumstances it might be necessary to use an assumed name for this purpose; if the target of the criticism is particularly powerful and the writer has some special knowledge such that others wouldn't be able to make the same criticisms, then an anonymous or pseudononymous attack may be proper.
Mr. Non-Volokh (who I generally enjoy reading) doesn't display any inside knowledge and makes only general assertions that William G. Myers III isn't worthy of being nominated to the federal bench. Mr. Non-Volokh may be entirely right, and his position doesn't sound baseless, but his attack could easily have been made by another person who was willing to go reputation-to-reputation against the nominee.