I don't think they are bad, per se. You've got to agree that they can be quite entertaining. I rarely comment in these communities, but it is always interesting to observe the reactions of those who post and comment, all the social interaction, and the slow shifting of views on controversial subjects, etc.
There was a rant a few days ago about incest in fanfic. The OP was surprised and upset that her interest in an incestuous pairing was being condemned and ridiculed, and she didn't seem to get why people would take such a stand against it. In fact, she was downright astonished, and in full rant mode.
It really is interesting then to analyze it from a cultural point of view, even though the topic, and the following wank (or discussion), does give, as you said, the fandom a bad name.
Wow, that was long, and not wholly appropriate, too, but I think I'll leave it as is.