In almost all cases where a car is within 50 feet of you (maybe more, I haven't studied) at night, the reflector is probably the brightest thing he can see. It's powered by his headlights, which are very bright. There are situations where the reflectors don't work, but most times they do, and they're the only thing that's going to work if you run out of battery power.

Reflectors are cheap and there's no good reason not to have them on. I run a 3.5x1.5" DOT truck reflector on the back of my bike. I think it cost about $2.

I don't know and don't care about legality. I know and care that reflectors work really well. The things that come on bikes are pathetic little things, I replaced them with bigger, much better mounted (on aluminum brackets bolted to the rack) reflectors right away.