Right. I say "no personal attacks" is a baseline rule for any rational endeavor. There are often (too often, in email!) borderline cases and so forth, but even so, personal attacks are just not ever the right thing to do on Wikipedia.
The trouble with having this as policy is that it is very subjective. Things like "I don't think you're qualified to comment on X" or "No one in this field takes Z seriously." or even "I think you're too emotionally attached to this subject to be objective" are perfectly valid arguments, but are likely to be seen as personal insults. We also can't ignore the fact that there simply are human beings here likely to erupt now and then, but who are useful contributors most of the time, and so we would naturally be percieved as "unfair" if we applied the policy laxly to those folks and more readily to those we didn't think were useful contributors.
I think we should be careful to make it clear up front that any policy on abusive language is, by design, subjective and applied only in extreme cases and only when human judgment deems it necessary.