I know plan 9 has factotum, but "google it" sure doesn't help me find a way to use it to authenticate windows clients. Or authenticate anything other than plan 9 for that matter. Aquarela doesn't even claim to work, and it is actually much worse in practice than it claims to be. And it can't be part of a domain can it?
I'm not trying to be critical here, but when I ask "what can I actually do with plan 9?" I am genuinely interested in a serious answer. Stuff like "well there's an unsupported kinda buggy thing that doesn't actually do what you need anyways" is just a less honest way of saying "you can't use it for anything".