My own U1 is programmed for CLS/AWLS work, with a button recalling the flash commander mode page, and anothet acting as Fv lock. Manual exposure mode, as recomended by Mr. Strobist.
U2 is set also for manual mode but with auto-ISO, used for lighting in difficult conditions or tele work. Choose the acceptable shutter speed according to subject motion and focal distance, set aperture as desired for DOF, ligth gathering or sharpness, and auto-ISO will take care of the correct exposure (within its latitude).
This is one of the great features on the D7000, the ability to program user modes.
Other possible uses are setting one up for black and white, action shooting, or low-light optimization.
http://egozarolho.blogspot.com 1. Good content, good aesthetics and good tecnique. On that order. 2. Light is more important than glass and pixels. 3. In the digital photography process, software is as important as gear.