Glass is not an augmented reality device.
I've worn one (very briefly) now, and let me be very clear: you can not superimpose labels or images on anything you see in daily life with Glass. This is a physical limitation of the hardware, and it is absolute. This hardware will never, ever, ever be able to do that.
And it's unclear to me if the hardware for Rainbows End/Halting State augmented reality will ever (or at least in this generation) exist if Glass-like heads-up-display hardware can not be successful in its own right.
There's also a little Easter Egg in it that lets you see the entire Glass team, and look around at them, up, down, etc. and it's pretty engrossing, so you can use it as limited VR as well.
(I'm playing with it as an augmented reality device and have written a calibration library to get the display aligned with the camera - see https://github.com/matt-williams/Optometrist. I've been working on hooking this into OpenCV which I'm hoping to upload in the next few days.)
One valid answer to "why not" might be power/heat - if you run it for a long period of time, your battery dies and your hair sets on fire. ;)
The camera not being centered on your face is not insurmountable. (In fact, you'd want it to be centered on your right eye, not your face.) The project I linked to above aligns the camera with your vision. It doesn't currently work for different depths of field, but I'm working on that - this relies on your image recognition being able to determine the depth of the recognized object, e.g. by knowing its physical size.
Why would anyone want to wear glasses to see text-messages instantly?
I use to be a glass enthusiast, but this single limitation is discouraging.