My rifle's bolt has been pitted for years and years, and the pitting does not seem to hurt a thing. Of course, it would depend on the depth of the pitting.
As for what causes pits? It is a weakness in the cup, allowing the hot, high pressure to blow out a small in the primer.
What did it for me was a carton of Rem. 9 1/2 primers. I decapped a few of them and sent them to Rem. Rem. did have the decency to write me back and explain that they had revamped the machinery that makes primer cups. This was to eliminate a step in the manufacture of the primers, but it caused a weakspot in the cup. Thus blowback and a pitted bolt face.
