Warning shot?

I read thread on Glocktalk about a scenario where a guy with a ccw came up on two men beating up another man. He was getting beat pretty bad and there were girls screaming and what not so he yelled something at them. At that time they stopped beating on the other guy and approached him. Supposedly when they were at 15ft he told them to stop and they kept coming at which time he drew his weapon and ordered them to get on the ground and he held them while someone called the cops. There's more to the story but someone said that it might have been a good idea to put a warning shot in the dirt in front of the attackers to prevent them from advancing toward you. I don't personally think this is a good idea but it got me wondering, is there ever a scenario where it would be "ok" to fire a warning shot?

Here's the thread on GT for those interested http://glocktalk.com/forums/showthread.php?t=1053301&page=2

-Frank