Hi Russell,
my 2000 K11 has exactly the same phenomenon. To my initial surprise, because I had an older K11 before this one, and in the old one the middle two had controllable temperature just like the side vents. This made me conclude that it might be intentional, so a design change.
The advantage in the newer model is that you can warm your feet while still have some fresh air in your face. This was impossible in the old version.
But I agree that I sometimes also would like to get warm air from all vents to warm my hands. Other than that, I've become completely accustomed to and happy with this newer set-up.