Well if we're going to get a plug-in that allows you to log in with your email, we might as well get a feature to sign in with Facebook and the likes as well, no?
I understand your concern though, but as far as I know, it's never been a problem. Plus, NB remembers your username like FB remembers your email and so on, so unless you are on a different device (mobile, etc) or on a different computer, it shouldn't be much of a problem.
As for security, you strike a point. Then again, this is NB and not something that is likely to be attacked (although it has happened before, but it was hacked all together, and that's a different story).
If it's a simple plug-in though, I don't see why not. In this case, Zise is the man with the knowledge and a reply to this question would have been more efficient if he was PMed instead I suppose. Then again, that wouldn't create the same "buzz" so to speak.