I'm fine with features using JS. It's just important that the feature is either an additive feature that is not necessary for use or it has a way to work without JS. In this case we're talking about login. If you disable JS... you can't even log into your own user account. And disabling JS is supposed to be an improvement to the security of logging in.
OK, here is the misunderstanding--if we implemented BrowserID, it wouldn't be the *only* way to login. It would be like OpenID, in that you could login with either a MW login or with OpenID if you have it. If you don't have JS enabled, we'll put in noscript tags that explain what you're missing.
Sound OK? Admittedly this is all hypothetical, but in case the Foundation wants to pursue this, I don't want there to be huge misunderstandings :)