Yes, I do. Often for several days until I realize I should probably put it in the dishwasher and get a new one out. I even at this point reuse cereal bowls/spoons (but always thoroughly rinse both before placing back on my counter to use the next day )
All of it is so that I don't have to run the dishwasher as often. Save on the water and all of that.
Oh.. and I've been using the same mug at work for hot tea for the past year, and I haven't taken it home to wash it since about a month into the job. I suppose I should think that's disgusting, but in reality I don't think there's much to worry about.