Well, after mouthing off about how I'd never seen Arcade equip the regular ol' laser rifle, I finally saw him do this yesterday. Him, Boone & Cass were on the top of a ridge near a police station when 3 bandits on the roof + 4 further away on the road attacked. For some reason all 3 companions (for once) stayed crouched on the ridge while firing...even Arcade didn't move forward. I could see the green from his default defender as he picked off the closer baddies from the roof, but then as I ran back up the hill I saw him stand up, pause a moment, then pull out the rifle and help out with the ones on the road.
I got it on video too.
Still, the thing for me is I want him to use a rifle 90%/all the time, not just every once in a while when he feels like it.
The AER14 seems to be working, so I'll stick with that.