I think you could argue this either way with security training vs. destructive tendencies vs. PTSD risk - but one of the core competencies is experience with risky situations and a proven history of not leaving post during an emergency. Much of the value of a security guard is having a level-headed authority when something goes awry - whether it is a fire alarm, a police officer inquiring about the party, a boisterous crowd in line, or a person who needs to be removed. Think of it as a "designated driver" as much as a protection.
They'd generally be more disciplined and physically fit / trained for combat. As long as they're screened otherwise, it seems like a great pool to draw from -- plus it helps out veterans.
The military are well drilled in rules of engagement and waiting for fire orders. Far more so than your average bouncer or off-duty police officer. I heartily wish the police force were trained and disciplined like the military, it would remove a lot of the individual variation in responses.
I'd imagine it would depend heavily on the military specialization and experience. Some specializations, e.g. military police, would probably feel right at home doing this. It's also worth noting that many members of the armed forces have served in peacekeeping capacities and probably have more experience defusing dangerous situations than your typical bouncer.
