I don't think it'd damage the computers, if it did, it'd be very minor, like you said. I'd say the biggest effect would be people's electrical bills being higher.
Bitcoin mining on Laptop GPU is known to destroy whole machines (usually by overheating several parts of the motherboard and ruining them... also ruining the GPU itself too)
I did some GPU mining for about 5 minutes once. Had to stop it immediately because the noise my GPU was making definitely wasn't good for it. The fan still rattles a bit sometimes from the 5 minutes it was being run at 100%
GPUs are not really designed for 100% load for prolonged periods of time