Dirty coffee mug? Or clean?

I have, on occasion, seen people keep and re-use their dirty coffee mugs on a daily basis (for either coffee or tea).

When remarking upon it, they seem to claim a discernible preference to want it that way. That is, it’s not simply that they’re too lazy to wash their dishes.

I’m sure we all could come up with a thousand reasons not to do that… but I’m curious if anyone here has a predilection for dirty mugs… and why? What makes it preferable?

If you could have your way, would you want to keep the office coffee carafe unwashed as well?