Just want to clarify. We don't teach the players to not take it off, we teach them the proper way to do both. We do allow for them to make the decision, however. There are some of our students who feel more comfortable leaving it on.
IMHO, there is no drawback to leaving it on and the time it takes you to find the ball is that much quicker. Not all pop-ups are moon balls, and there are times when leaving the mask on may get you a step or two closer to the ball before it hits the ground.