My thumb that's putting another magazine into an empty magazine well, is an appendage that is doing nothing and is capable of filling the operation of dropping the bolt catch upon seating the magazine, and it happens to land just on top of the bolt catch/release when fully seating a magazine.
I would rather keep my trigger finger focused on being outside the trigger guard until I want to press the trigger, not thinking about activating a lever that is passing through the trigger guard to drop the bolt; an action which my support hand is more than capable of doing, and very quickly and smoothly when trained.
I don't need to train with one to know they are not for me. But to each their own, glad for Magpul that there are seemingly quite a few people that prefer having them.