Interesting discussion - I've certainly learned a lot. Just to give you a quick overview of my background, I supported rotary wing ops in Afghanistan and I currently work with UAV's in both theaters.

This, I think, says it all though:

On the arrival overhead of the fire force the insurgents would become disorientated by the noise and firing and be looking for a way out. Insurgent fire was then less about aimed shots and more about the actions of increasingly desperate men.
Well, that's a huge difference with Afghanistan. A lot of fighters in Afghanistan have a hard-on for bringing down helicopters. They've used IED's with the specific intent of luring medevac/qrf helos into a kill zone. They sometimes lurk near FOBs and try to take out helos that way. In short, for the most part (it's hard to generalize about Afghanistan -circumstances vary widely depending on location) these are not fighters who become disoriented and desperate when helicopters arrive.

Besides that, a combination of factors make employment of those tactics difficult in Afghanistan. Altitude and air density has already been mentioned, but there is also distance. Engagements are usually over long before a QRF would arrive. They've seen nine years of our CAS capabilities, they have a pretty good idea how long it takes for CAS to arrive on station, and they often (though not always) bug-out before it arrives.

We also don't have native "scouts" with intimate local knowledge to find targets for us. UAV's are a poor substitute.

As a result, it's pretty rare to find a group of insurgents loitering in a place where such tactics would work. Most fighters are part of the population and stay close to the population. The exceptions are in areas most hostile to helo operations - the mountainous border especially.

So, it doesn't make much sense (to me at least) to devote limited rotary-wing capability for the relatively rare circumstances where these specific tactics would work. And if we did, the enemy would soon adapt and set up ambushes for our helicopters as they have done before.