If the device is going to live somewhere away from a PC ou need to power it, but for the occasional trip to the PC to be updated or whatever it comes in very handy not to have to go searching for the wall art.
What I proposed doesn't prevent that, it just requires moving a jumper clip from one position to the next position. What's not handy about that?
Another example of the auto-voltage lameness. Assume your project is powered externally and is independent to a PC. Why power up the FTDI or 8u2 chip? Check out the Seeeduino design for power selection, manual two position switch for usb/external power and FTDI power source is hardwired to the USB connector only. That's good functional design.
Lefty