Never use it although I've tried. The amount of time to focus on a passenger side mirror seems dangerous to me, especially in heavy interstate traffic where quick lane changes are required. A quick turn of the head has been more than sufficient in most cars I've owned. There is a trend towards ever poorer rearward view with blind spots on the larger rear pillars. Cameras will eventually compensate for that (and remove all need for external mirrors).
My current car is a PT Cruiser, purchased new end of 2001, which does not have a great rear view, but better than many current new cars. New, it had a bit of a funny car jacked up rear end. I modified the rear springs to drop the rear end two inches. Not only did that have the desired aerodynamic effect by removing a full two inches of drag area, but it also improved my rear view immensely as an extra benefit. Then I removed the passenger side mirror putting a matching piece of black vinyl in its place. I compensated for it by putting a larger inside mirror. It gave me a full view of all passenger side windows and the rear window, virtually eliminating the blind spot. Never had a close call of any kind.
So the PT Cruiser does not have great fuel efficiency stock. In 2001, EPA ranked it as 21/26 manual transmission. Besides the two aero mods above, I took out all the removable floor mats that came with it (amazing how much they weigh), and the rear seats. After 180,000 miles, my overall mpg is 28 mpg, with lots of cross country trips at 70 plus (not many local trips - bicycle instead). Not bad compared to many cars, but by far the worst of any car I've owned after 1975. So I'm looking to replace it and the Niro looks pretty darned good for me.
I've never bothered removing one of the two rear mirrors on my motorcycle although having two on a motorcycle is completely superfluous as visibility is much better than any car. A ten degree rotation of the head gives you a clear view of the blind spot - I only use the mirror to watch rear traffic (habit from driving 75 during the 55 years looking for traffic cops). Aerodynamics suck on motorcycles but that is a feeble excuse. I should remove one of the mirrors as I'd probably notice the gain. Never thought about it until you posed the question. I bought the bike a few years ago to save money on local trips. Not sure it really saves money, but it does get 68 mpg over the 13,000 miles of ownership. Saves time too as it is easy to pass slow movers on two lanes (hugely overgeared for fuel efficiency and I sometimes have to drop three gears when passing). A Honda NC700X if anyone is interested. No other "big" bike gets anywhere near that. One guy on Fuelly has a lifetime average of 76 mpg! But I ride briskly and around 70 on the interstate.