My money's on the guy who shoots first. Grenade launchers don't tend to be particularly useful if you're unconscious and taking effective fire.
Anyways, IRL grenade launchers also tend to be pretty OP as infantry weapons go... in a vacuum. They tend to be a tad hard to use around civilians, priceless artifacts, etc., however. There's also a lot of extra potential for them to go wrong. With a bullet, you need to check your background insofar as a brick wall. With a grenade launcher, you need to make sure that wall has legit armor plating before you go turning a chunk of the building into a bunch of frag and pelting your buddy on the other side with it.
As for why it's a +2? I'm guessing it's for quick handling of the area of effect in a theater of the mind's eye engagement. They do talk about horseshoes and hand grenades after all. Personally, I'd do what Alpha Strike does and swap it to a -2 or nix it entirely for mapped play.