Also, more blades on a rotor system must mean that they can "slipstream" behind the other blade to some degree, and therefore have less air resistance per blade as compared to a rotor design where the blades can't "hide in the wake of the blade ahead of it", as in a 2, 3, 4, maybe even 5-blade system?
Adam,
It's not quite like drafting a truck with your geo metro. First, the induced airflow is from the top, the wake wants to depart downward, so you can't hide one blade behind another. Unless, of course, your solidity is 1.0, but then there is no place for the air to go because you just have a solid disk.
And, when one rotor does hit the wake of another rotor, usually there is vibration associated with it. And if there is vibration associted with it, you can bet it is causing drag and loss of performance.
-- IFMU