I don't think I get it; you don't have to set the board to USB power just to connect it to USB; if you're developing a battery-powered application, why don't you want to run off the battery (or other external power) ALL the time?
In my specific case it's because of power constraints (battery, not wallwart). Whenever I have to take the Arduino from the development environment to the project's final place (a bicycle), I unplug the USB, place the Arduino in its cradle and plug in the battery. When after testing there's the need for any changes, I unplug the battery, remove the Arduino from its cradle, and plug it into the USB port. The Arduino is wearing a shield, so the jumper is not readily accessible. A power switch makes all the difference to me
With mellis' post, I hope there's an even easier solution coming up (automatic power source detection?), but for the time being, the modification I made is simple and quick to do. Even if "you don't get it", maybe it's of some use to someone else