I think Saintsman is asking: why not use a propeller instead of a rotor?
(Identical function and drive; one is just a lot smaller.)
The rotor dynamics experts can probably tell you, but basically to have a much smaller "rotor" (ie: propeller) support the weight of a helicopter it would have to have a lot more power pumped into it, for any given weight, than a rotor designed to support the same weight.
Rotor/propeller...they are both aerofoils. Long and skinny (like a glider) or short and stubby (like an F104.) And with different characteristics.
Too small an aspect ratio, it drops out of the sky at low airspeeds, too high and it's hard to build the required strength into it. (Like that round the world non stop job designed by Bert Rutan..they didn't want to exceed 2G on that wing.) I'd imagine rotors behave in a similar fashion and with similar design limitations.
The Hughes 500 has a fairly compact rotor for its mass, makes it ideal for getting into small spots, and very maneuverable. Is a more extreme example of that what you are thinking?
One problem I can see immediately with that configuration (conventional prop above, acting as a rotor) would be the auto-rotational capabilities of it, should the powerplant go all quiet. It'd probably glide in a more brick-like manner, and the window for successful flaring and touchdown would be nanoseconds.