Earlier quoted context omitted.
It's derogatory because they used it in a way to imply that they didn't particularly want to be associated with the group labeled "nerds." I've always preferred the term "geek." Anyone else feel the same way?
I've always thought "geeks" were Mac users (that was the crowd from which I heard the term used most, back in my Amiga days), and nerds were nerds. That said, I may call myself a nerd, you you best not be calling me a nerd. ;-) Seriously, though, for a group of people who like to call each other out on standards of conduct, it's pretty appalling that they used a derogatory term to jokingly dismiss their utter lack of…
But I agree about the way that either can be used as positive expression of self-regard, while still coming across as derogatory when used by people who are not in the club and unsympathetic towards those who are.
In this regard, both words differ from 'dork', which never seems to be a good tag.