Back to spinners, there has to be all kinds of dirty air created from the gaps between the prop and the spinner cut outs, plus any other access holes, etc. that are whipping around. The FAI pylon racers' one piece prop/spinner with all blended surfaces isn't just done this way for looks.

I think the idea to draw air through the spinner for cooling is a crafty way to get to that goal, but I don't believe in any free lunches...something always gets lost in any energy exchange.