Yeah, when I was initially designing this I was doing it for my own purposes and was thinking it would be silly to power the Pi off the same supply since if the power went out then you wouldn't have a way to know since the Pi would loose power too.  Sort of realized after the fact though that there were a lot of different ways people would use these and probably should have put a 5th pin on the header for 5v power and offered a USB socket add-on board you could pop on the header.

If you have a soldering iron you can actually mod the board to add a USB power socket w/o too much trouble.