Right, headphones still aren't appropriate in many situations. You can wear them on the subway (although a pretty small minority of people do).

But try showing up to a meeting at work with headphones on. Or to a family gathering. You would instinctively take them off immediately.

The presence of headphones is still a social cue, and the presence of Glass will always be one too. It's not going to become just a neutral thing like wearing eyeglasses.