MR is horrible for one pocket, or straight pool, it would get in the way of too many shots since the balls are in the rack area for much of the game.
Even if a ball sits on the rack in 9 or 10 ball, I usually remove it from under the ball after getting the OK from my opponent. Just lift and slide.
The best game to use the MR rack in is 10 ball. It's great in 9 to keep the 9 from going in the corners or from heading there and hanging up for an early combo, but it makes pattern racking and a soft break too powerful.