Sure, we should judge posts based on content, but someone's motives should be taken into consideration with sock-puppetting. Why does someone want to say something that they don't want to attach their name to? I'm not saying there are or aren't good reasons to do this, but I would be suspicious of comments that someone insisted on making anonymously.
Another problem with sock puppets is that these sites run on the principals of democracy. Allowing multiple accounts ruins that.
It seems to me that posting under multiple names, if you want, is a natural extension of anonymity and privacy.
(Note I do not "sock-puppet" here. I asked PG and he disapproves.)