On reading this topic, I was confused about the 11 point AF selection that I tend not to use.
Thom Hogan, Complete Guide to D800/E, pg 571 explains it this way. Select 11 points for one reason only, speed in selecting *initial* focus area. As Roy said, you still have all 51 sensors available for use in Dynamic Area AF. That's fairly difficult to figure out if you just look at the camera; you are still free to choose S, 9, 21, 51 pt dynamic areas, they are all still displayed in the VF, but when you select, only 11 sensors are available. They *are* still available for dynamic subject tracking though.
However, as Eric points out, you're reducing the number of cross sensors available. Or as Thom calls them 'cross-hatched' sensors. Thom indicates there are 3 cross sensors, the center column, when 11 point AF is selected.
It's a real trade when you are photographing rapidly moving small subjects, e.g., BIF. Why reduce the no. of cross sensors available? Initially, my thought was for tighter control. But I'm only able to control the initial focus point selected, and my penalty is a reduced cross sensor function set. This doesn't sound like a great trade for BIF, perhaps other application areas - other thoughts?