I have a little blue bubble light that I use when I'm stopped along the road at MVA's or to mark a loctaion for the ambulance... BUT I learned never to leave it up there when responding. Hit a bump and that ba$@#%d came uh... Unmagneted, or what the fudge ever, and smashed all over the road. It's just not worth it, you can't (or shouldn't) exceed the speed limit anyway; the other cars are going as fast as you are (excluding flatlanders and old people). So why do you need one for that?
Not trying to offend the whackers, but the only use I see them fit for is being parked along the road to get the attention of other drivers so they don't plow anyone down.