I don't think that these women are doing such a thing *just* because women are underrepresented on the Internet, I think they are just letting interested women know that the stories aren't necessarily true, that you *can* be a female geek.
They're giving women a support network for when things do go wrong, when they are discriminated against or whatever. Even really really geeky geek females have probably been discriminated against or somewhat abused at least once in their lives by other (usually male) geeks. I'd classify myself as mostly a geek under your qualifications (I do have another life outside sitting at my computer, but I do the latter *quite* a lot), and I certainly have - several times. I don't think you can call it (LinuxChix in particular, of which I am not actually a member) a sisterhood as such, the LinuxChix welcome men to their meetings and on their mailing lists, so if my understanding of American culture's "sororities" is correct, this is somewhat different.
And you *are* being elitist if you say "90% of the engineers I've met are idiots" - and frankly, it's not very productive to be so openly judgemental about people, especially people you may be working with.
In Australian culture there is an idea that you should give people a go, whether or not you immediately think they're stupid - you may be pleasantly surprised. If a woman, or man, or anyone, thinks they have the mindset, give them a chance at that - if they fail, then they'll work it out and do something else. I think the most important thing is encouragement and education, and these groups provide it for women who may not be able to get it elsewhere for fear of ridicule or otherwise - and some just might end up finding that they really are good at coding or sysadminning or whatever, you never know.