If you read the story about "Blackhawk Down"...
you'll find that such a system could've been useful on those helicopters but I suspect it would've been saturated by the sheer volume of RPGs fired at the Blackhawks. From what I remember, dozens of RPGs were fired at each helicopter. The pilots were able to dodge most of them but an RPG is able to punch through several inches of armor, so a single hit was enough to inflict severe damage on a thin skinned helicopter. Better than nothing but not a complete solution by a long shot.