I don't know that they do degrade, over and above normal wear and tear. I suppose they must, to some degree, like anything else. Doesn't hurt to maintain them, though. Some mics I've come across in the past, well, it's a ruddy marvel they worked at all, seeing as how they had years of general crud (spittle, beer splashes, dust, etc.) Of course, these mics were real road warriors.

Unscrew the mesh, take out the foam insert and rinse it gently in warm soapy water, then let it air dry. That, at least, gives the mic a fighting chance if done on a regular basis.