It depends ..
I leave mine on Full Auto all the time. I was wondering about the same thing until yesterday when the middle vent blow "strong air" at my face. The weather has been very cold about 30 degree until yesterday when it turn above 50 in the day. I figure the MDX has a certain logic about this Full Auto thing but I don't know what. If somebody has a "flow chart of logic" please share.
Along the same line, I feel the back is too warm when the car is fully loaded. When I leave the front to 63 degree, with six people in the back, the back air is not cool enough. But again, when I am by myself the difference is not notice-able. Of course, the separate AC control unit in the back completely takes care of the problem.