That armor helps negate the AP and concussive properties of projectiles by having the detonation occur not right on the surface of the armor. Still, all the super fast and lethal shrapnel peppers the armor. For example, you slap the fence on a -60 even though say an RPG detonates farther away, all that shrapnel is still going to "hot knife through butter" the un-armored sides of the helo. Still going to wreck shop on engines, personnel etc. Doubt the cost benefit is there.