First, vision is much higher bandwidth. You can glance at a notification and immediately know that you have a message, who sent the message, and how you received it. Takes half a second. Bluetooth earpieces are only suitable for sustained conversation. They are social faux pas because you are having a sustained conversation with someone else. This is not the case with HMDs - someone using a tool like Glass is actually more focused on their task and less distracted overall because they spend less time switching contexts. Citation: http://dmrussell.net/CHI2010/docs/p1695.pdf
Second, glancing at an HMD is fundamentally different from pulling out a cell phone because you're not looking away from the other person. It is difficult to emphasize how important this is to people who have not done work in this field. It is looking away that breaks conversations, not doing something else. Conversation continues to work as long as the other person thinks you're looking at them. And having an HMD on is close enough that people don't react to you jumping away into glass-land to look something up or jot down a note. This one doesn't have a citation, but I have spent several years taking classes from the guy who now runs the Google Glass project, and I can tell you this from direct experience.
I've said it before and I'll say it again: please, please try these things before you dismiss them. Social interaction is sufficiently complicated that experimentation is the only way to predict how people will react to something. And the experiments show that Glass works.
