I first encountered the practice of never washing a coffee cup, when I was sternly warned, and nearly physically threatened by a former employer to never wash his coffee cup.
Apparently it is an old military tradition to never wash out your coffee cup, and being a retired service man he continued the practice.
Is there any logic behind this, does it noticeably effect the coffee, or is this just an old, superstitious tradition?
The origin of not washing your coffee cup stems from the age when coffee cups were made of metal. An example can be still be seen with the Italian practice of seasoning a moka pot. When a metal coffee container (moka pot or coffee cup) is new/washed, the coffee takes on a metallic flavor. Once the container has been used once or twice, the oils from the coffee seal the internal surface preventing the metallic flavor from tainting the brew. (Many Italians believe that a new moka pot is not usable until several pots of coffee have been brewed and dumped out first)
It is easy to imagine the evolution from practical process to tradition by thinking of the time of transition from metal to ceramic coffee mugs. The young service members who had not used metal coffee cups would be used to washing them, while the old senior leadership would still be in the habit of not washing them and probably had developed a taste for it (hence swearing that it tastes better when the mug is "seasoned").
It may be superstition but many people, myself included, don't wash out their coffee cups. I rinse out my cup but don't wash it - I don't want soap going into my favorite coffee cup. I like the way it smells of coffee. I don't have to see or touch my favorite coffee or tea cup to know which one is which.
In addition I never use soap on either my coffee or tea filters.
Would I notice the difference if I had two identical mugs - one with the "patina" and one without? Maybe not. But I prefer it that way.
Could be a way of "marking" your mug by way of the stain suggesting that the mug is in use. This only works for coffee mugs that are tied to a desk.
Consider that in a restaurant, mess or chow hall, mugs filled with coffee are washed and folks who drink their coffee from stained mugs will drink coffee from clean ones. I know I do.
At work, I drink from a stained mug. Why? Because I'm indifferent to it and kind of lazy. But at home, we put our coffee mugs in the dishwasher. I get no sense of a difference in taste from cleaned or stained.
I think it just comes down to personal preference and has nothing to do with logic. Most work spaces are fairly impersonal, so injecting some ownership over something even as small as a mug is worth something.
BTW, why would you bother to try and wash a co-worker's coffee mug?
Patina, may be dangerous if the cup is made of copper. Patina is a formation of oxidation, and the greenish copper oxide is poisonous, please get rid of copper cups or make them plated with tin as in traditional Turkish cezves. (See this related answer.)
Another perspective may be the smeared oil and its contribution to the overall flavor (or at least the the oil prevention of metallic taste). It may be beneficial for a few days, up to a week for example. Then, the oils will start to rot and add a sour taste to the flavor. (See this answer.)
