I guess that might be handy at night when someone behind you had headlights on high shining right into your mirrors.

Not sure about all states, but here you weren't supposed to have a spot light on a vehicle that wasn't 'Official Use'
One of my uncle's mentioned that to another uncle that was a small town cop after WWII, then chief for many years.
He said 'Oh yeah, that's not legal at all? Who is it?'
When my other uncle said the name, he responded with 'Oh him, He's OK. I gave him a note allowing him to have it on his car.'
(it seems the note was in case another cop or state trooper pulled him over.)