That's the theory, but in practice the plastic props break well before harm comes to the engine. At least that is true for glow engines up to the 1.20 size using APC, Master Airscrew and Graupner props. If you have a big gasser or some unusually strong brand of prop, things may change. I've bent crankshafts in crashes, but only by hitting so hard that what the prop was made out of had no bearing on the damage .

Oh, I forgot, I mostly fly off of grass. Grass is more forgiving of noseovers and dorked landings that result in digging a divit. But I've also broken plenty of props off on other things besides dirt with out a problem, so I don't think pavement will change things much.