Current devices in the car cannot cover the centre of your view with some graphics. In most cases they're designed so that it's actually impossible. Can the same be guaranteed for Glass? I'm not even thinking about notifications and apps. What about a random app crash / reboot - how will that affect your visibility while driving?
Ok, not the complete centre. The display I've seen in all videos takes up around 1/9th of the image - I'm not sure how it translates to normal field of view. But imagine that 1/9th of your view starts flickering - that would be rather annoying and noticeable, right?