Illusion of safety. It's the distraction that really matters. Of course looking down introduces a risk factor, but studies have shown that it's the distraction itself that is dangerous. The HUD will create the illusion that it's ok to do certain things -- more often probably too -- while driving, now that you don't have to look down to do them.

One could also argue that when you have to look down to do something distracting, at least you are being reminded -- by taking your eyes off the road -- that the thing you're doing is distracting and you should probably pay attention to the road.