What happens is that charge blowing past the piston rings will fill the crank case and push intake air back out... Since the charge is burned already (has no fuel in it) it acts like an air leak, heats up your piston, and messes up incoming charge.
Reed valve engines don't care so much, I had a hobbit a couple years back that was like throwing a hot dog down a hallway, ran great! But piston port and rotary won't run at all with some blowby.