The reinforcement is needed for pop rivets because the small round "shop head" can pull through the fiberglass quite easily. The nut-plate has a much larger footprint. So the reinforcement is not needed.
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School RV-12 Project Mentor.
RV-6 Flying since 1993 (sold)