I wouldn't consider a private recording a "Big Brother" thing. It's when recording is mandated and that you have no choice in sharing it, that it becomes "Big Brother"-ish.

Personally, I could see a passive recording feature (say, the last five minutes) as useful in this way. But then we're talking battery life, and I haven't read enough to know about such things for Google Glass.

"crime witness aid for police" would probably be beneficial by itself, if it is possible to limit the privacy and surveillance concerns. So essentially the situation, where a witness starts to film a crime and then gives this video to the police. Or the police could be required to document everything they do (by taking a full video of each shift).

On the other hand, the surveillance potential is of course scary. And once Google glass is out, I really do not know how to limit the impact.