slgrossman,
You are right on, all those blade condition factors I imprecisely lumped into the word erosion. The drag of the blades will directly rob power and performance, to a degree greater than the typical bug-smash's effects on a light airplane. The rotor tips of even a light helo are moving at about 675 ft/sec, or about 360 knots, where the typical light airplane moves its wings at 110 knots.
I can believe a 1"reduction in MP going from dirty to clean.
barranfin,
Frost is a real problem, dust perhaps less so but still not good. Never fly with such pervasive stuff on your aircraft, especially lifting/control surfaces. It is quite alright to see pure frost on preflight inspection, run up and pull no pitch, then shut down to see if the frost has sublimated. Never do this with ice or thick frost, your neighbors on the flight line will get rich from the insurance settlement from the chunks you hurl into their aricraft. Your tail rotor might also take a pounding.
More insideous and more unsafe than the loss of performance is the loss of max lift (at high angle of attack) that you might get from dirty airfoils. This could lead to premature retreating blade stall and loss of control. I had a flight in an old S-58 where the blade erosion (which was ageneral sandpaper condition on the abrasion strips) lead to blade stall on gusty afternoons, and a 10 knot cruise speed reduction. Swapping blades made everything better. On a tail rotor, this could lead to loss of anti-torque margins that might embarass you at the wrong time.