I've kept them on or added them to bikes I have. I figure the more visibility, the better.

A while back, I was assisting at a 48-hour bike race, and that included driving the sag wagon the opposite direction from which all the racers were going. One thing I was surprosed at was how non-distinctive a bike headlight is. I'd see a bike headlight off in the distance and see it for several seconds before realizing it was a headlight of any kind. They're not the same size as car headlights, and not paired like car headlights. Anyway, the more light you reflect, the better.