Do you think burning candles is any worse than letting the musicians smoke?
Cigarette smoke is supposed to be about the worst thing for analog tape. Leaves a sticky residue on everything, dirty as hell. I imagine marijuana smoke isn't much better, as I recall the wonders of reusing resin in a pipe. Why do you think so many ebay listings make a point that the gear was in a smoke free studio?
For the whole issue of is it bad for the mic, none of this stuff is ideal, nor is a singer who just had lunch, but some people in the mic biz will do clean-ups. I think SPA will clean grunge of the diaphragm as well as do other checkup/restoration stuff, and you don't have to do a rebuild to get them to clean your mic up. Probably worthwhile for any high ticket vocal mic in a busy studio to get this treatment at some point.
Bear