As most psingers will tell you, there are few things more pleasant than a fresh pop-shield or grille on a microphone. Since it happens very slowly, it can be hard to spot the gradual deterioration in frequency response as a mic grille becomes worn, bashed and clogged with years of punk spit and dust.
Simply changing these from time to time can add a considerable amount of lifespan onto your mic collection, even if you are dropping one from vocal duty to, say, snare or guitar amp. It freshens up the top end frequencies as well as making for an all round more pleasant experience for anyone forced to get their mouth close to it.