I remember doing the MOB drills that you talked about, and I too was confused as to why you circle (in my opinion taunt) the victim. The reason that I was given (decide for yourself)is that the backwind style MOB is designed for use with minimal crew, meaning simplicity/safety is the goal. If you simply backwind and come alongside the victim, the boat is coming downwind toward the target. If something is not quite right, or the helmsman has to be the one to get the victim, there is a possibility of the boat being "pushed" into the victim. If you complete the circle around the victim, you approach coming upwind. It this situation, if anything goes wrong, the boat will be pushed away from the victim, lessening the chance of learning what keel hulling is really like.
This is what I was told by the seminar''s instructor, and after watching some downwind-gone-wrong versions, it seemed to make sense. Not much, but some.