It leaves a very nasty film on things; the interior of your lungs, your skin, the body and diaphragm of the mic (the diaphragm is what you really want to protect) basically anything it comes in contact with. Ever noticed how your clothes and hair smell after a night in a smoky bar? Your mics can acquire the same odor and you don't want that residue/film on your diaphragm which would take a visit to a qualified service center to remove. Pure speculation on my part but repeated exposures to smoke could add a tiny bit of mass to the diaphragm and subtly change the characterisitcs of the mic as well as acting as a sticky trap for any solid airborne particles that encounter the diaphragm and trap them there, further changing the characteristics of the mic.
Having a father who smokes, I hate to visit as my clothes acquire that odor even though they hang in a closet at the other end of the house from the room he smokes in. The smoking room also has a light brownish tint to it the other rooms don't have and it's darkest directly above the ashtray on his desk.