It just seems to make sense to me. I cut out the housing and it is flat, as long as my cuts on the end of the housed timber are true, there is relatively full contact. I cut the mortise a little deeper than the tenon, but the bearing surface sets the length as well as provide bearing. The scooping doesn't seem to be necessary, and removing some of the bearing surface seems counter inuitive to me so I dont do it. I would to improve an ill fitting joint though. But usually, like I mentioned, I'm usually too busy sneezing after all that router work.
