It's a sure way to get pulled over fast, but I really can't see it being that dangerous. If he is shining the laser out the back window of a car, while he isn't driving, and he's basically pointing it at the sky, there is little risk of him hitting another car or person assuming any kind of high angle.
Confession time... I've done it on empty roads out of boredom. I mean if you're on a long drive on an empty highway in the middle of the night... getting pulled over is the only real issue. Haven't and wouldn't do it on a busy road.
As for distraction, I think a beam of light somewhere up ahead is less distracting to people than the usual crap they do, like talking on cell phones, texting, emailing, eating, drinking, etc..