The pins would surely abrade through the thin paint in time thanks to the vibrations coming from the GPU fan.
I'm not sold on the idea that this backplate does much for the GPU though. This smacks of those supposed signal booster strips you could buy and tape to your cellphone battery and magically it would enhance the internal antenna and your reception was supposed to improve.
I see a lot of anecdotal claims it improves cooling, but zero data or actual reviews of any sort to be had. I don't hold it against anyone, to each their own, but I'm not compelled to buy one as no one has shown me the benefits of doing so.
My favorite were those "Tornado" metal inserts for cars that were supposed to boost horsepower and gas mileage by putting them in the air intake.