The financial efficiency (or inefficiency) is a lot easier to understand than the aerodynamic stuff. I'm cheap (or "efficient" so I'll have to stick with two blades.
That is exacty what efficiency is, less dough per mile. We don't travel in R/C planes so efficiency is not always the upmost. People confuse effective with efficient. Using a glow engine is more effective than a gas engine, but it is not more efficient. Sometimes a prop may be much more efficient than another, but if it won't hover your 3D plane it's not going to be very effective.