Anonymity online: What good is it?

Seth Godin is said to have said that anonymous commenting is not a good thing:

"…Mathew, the forces for anonymous are running out of time. For almost two decades, the internet has been anonymous friendly, and what has come of it?…" (from GigaOm, via @davewiner)

Here's what Rosie O'Donnel said about Twitter, which gets to a similar point:

"…Correct, and that's what it is. It's like standing on a stage in a darkened comedy club and people throwing shit at you and you still trying to do your act…" (from transcript of Here's the Thing interview)