Teresa Nielsen Hayden on the blogger Code of Conduct:
You can’t have a good online discussion without moderation.
She’d know, too. Making Light is one of the most healthy and interesting online communities you’ll find.
For what it’s worth, I do delete comments. I delete anything that looks like spam, and I don’t tolerate the pointlessly obnoxious. Fortunately, I get very few pointlessly obnoxious comments, so I almost never choose to delete anything aside from spam.