Reality is not going to follow into the drills that we all practice. You can practice how to react in certain scenarios, but the real goal is not the explicit curriculum that is being verbally taught to you. It is the implicit that is important(what is not verbally said but an unspoken objective), and in the drill case it is teaching you to react to the threat and improvise. To be fluid to the situation and keep your head on a swivel.
I have to agree that having to react to an unknown situation forces you to make decisions while under duress which is good. This can also be accomplished, IMO even better, in FOF yet using more realistic scenarios.