I wonder how many accidents have been recorded which could have been avoided had the pilot shouted 'clear prop'? I'm guessing not many but let's have some hard statistics here.
Ah yes! But isn't the tenets of airmanship the process of reducing risk. As is costs nothing to yelp, then however small, the risk is reduced for no overall loss. A bit like strobes really
I have often seen people about to take a short cut across the patch where my plane is, gassing away, oblivious to me. A quick yell and they give me a wide wide birth. I have never felt comfortable walking near a running prop and neither do many people I wager.
I imagine in the old days, when starter motors were new fangled things but ground crew were still essential, there were many horiffic accidents.