Consider a semi-auto rifle with a floating firing pin. Each and every time the action cycles, the primer receives a light impact from the firing pin, and each and every time the rifle fires, it fires with a previously dented primer. These rifles, in their better examples tend to be extremely reliable, so previously dented primers do not seem to affect the reliability of the primer. What a dented primer might do is increase the ease by which the priming compound might be set off, as the cup, anvil, and priming compound are now in a lightly compressed orientation. An interesting experiment might be to repeatedly chamber a single round through an AR to see if after multiple firing pin strikes it fires on chambering.
I don't seat primers immediately after changing the oil in my truck, without first washing my hands. As a rule I avoid touching primers, but when I do I don't discard the primer, or segregate the cartridge its loaded in. I too did a primer test with WD-40 to see how primers were effected by contact with it. I put the primer on a steel bench and hit it with a hammer. In each case there was a small puff of white smoke, the sound was very muffled, and I saw no flame. This was after immediate contamination, and I didn't think to try the test after waiting for the WD-40 fluid to evaporate.