From what I understand the benefit of this type of "underarmour" is that it stays put better in a crash/slide as the abrasion resistant outer material (say Draggin Jeans, or in the case of upper body "underarmour", your MC Jacket) is free to move around a bit. In other words, the padding stands a better chance of being where you want it.
Too bad there is still no protection for "manbits", I have heard of some nasty injuries draggin the crotch area over sharply styled tanks & screens. I will say that the side/rear protection looks outstanding.