I know its been beaten to death, but here's my view anyway.

Bright, poorly aimed lights on bicycles are dangerous for this reason:

Not everybody has them.

If everyone had bright lights, it wouldn't be an issue. Most bicycles around here have lights. Hardly any of the pedestrians (and their dogs) do. When I've got your set-to-kill halogen beam scorching my retinas for 10-20 seconds, not only can I not see anything on the trail but you and your headlight, but once you've passed I have near zero visibility for a moment. That's all it takes for me to plow into some unsuspecting dog walker.

Yes, yes, slow down then. I do, but it's still a hazard. Even pedestrians that are responsible and aware can get injured in the process, not to mention me and my ride.