I can actually see this being of some interest for UAVs, not so much for manned helicopters for the reasons stated above.
Many of UAVs do not have a way to land after a mechanical failure, even if it is just a "simple" engine failure (example, all the quad- and octocopters out there). They are getting bigger and heavier every day, and they are being used, legally or not, over populated areas. A ballistic chute would probably not be a bad idea. Here is a good example why:
As an added plus, you wouldn't even have to blow up the rotors prior to deploying it.
As for your original assignment, there is a Russian attack helicopter that has ejection seats, including the exploding rotor.
Kamov Ka-50 - Wikipedia, the free encyclopedia