I thought of that also. One armed guard can get the drop on a perp, if the perp isn't aware of the guards location, or doesn't plan to take him out. I agree that a very well thought out plan would include taking out the guard, but many of these incidents are not very well thought out. Only a few were. Therefore, it is my opinion that an armed guard, as you stated, gives an appearance of added security that may thwart some attackers.
Dang, this has turned into an ugly thread. Is there another, somewhere else?