It could also have been a safety issue. While those face masks are sufficient protection for a single shot to the head, if they starting spraying the crowd with a full auto paintball gun, it's possible one of them may have failed.
IMO the way they staged it was appropriate because you'd want to conserve your ammo and try to make ever bullet a headshot, so a full-auto isn't as useful as you might think. Even if you happen to be standing there with a minigun, you're going to run out of ammo pretty fast and you can bet the horde will gain more ground every time you reload.
Plus for course every time you hit a leg, you can turn a walker into a crawler, which creates a new type of threat. Worse, if you hit the throat and sever the head you've just created a snapper. While immobile, they can still bite and present a nasty threat if you need to walk through the bodies to escape.