spark plug ?

[rod]

let's say you just made a great run, pulled the plugs and each looks just like you want them to. you check your log book and find that your predetermined number of runs on that set is at your limit. you go home and toss the plugs in the dist bin. next race, it's a new set. sound right?
OK. what i want to know is why did you toss them. if there was no physical damage and no miss fire, what makes them worse than a new set?
is there some chemical or electrical change which takes place?
i want a serious answer/discussion, not a 'just cuz i don't want to take a chance' answer.

thanks rod in AZ