I'm in the same boat as John Van.

My understanding is that the build up of sperm plugs has nothing to do directly with fertilization.....I would think though that it does indicate a few things to the breeder...

A.) the production of sperm (e.g. whether a young male could sire offspring)

B.) the rate of sperm production (based on the frequency of plug regeneration)

C.) actual copulation with the female (meaning if he has plugs after being with a female then most likely he didn't breed her even if they did appear locked at one point)

Hope that makes sense? And helps