Well, science tells me that the bacteria in boogers acts like immunisation shots. You know, small doses and your body learns how to fight it. So thats a no-brainer.
Skin is part of your body, so I figure that by eating it you regain the nutritional value. So no-brainer.
Scabs gross people out. So again, no-brainer.
And blood is just tasty. So you guessed it, no-brainer too.