I've had some mixed feelings about lights and sirens on the freeway. It seems like even when there are a few cars on the road, they always seem to group up and get in the way somehow. I say just turn them on as needed, but I wonder how people react when the lights are turned on for a couple of seconds, they move out of the way, you pass by, and then you turn it off, lol. People are dumb, but I guess what other people think on the road doesn't really matter in the end.
I use secondaries when I'm driving fast in the fast lane ER to a call hella far out into the desert, then light up if I need someone to move over, then back to secondaries when I pass them.