Batteries, OK, agreed. But...
@fungus:
For "power supplies", you might as well use a USB charger cable correct? Are there places where someone has an advantage using a different-voltage cable? (because after all, as you mentioned, it's being regulated to 5V).
( I do see one small advantage: As @Retrolefty mentions in the following page, with a different power source, you could potentially draw slightly more current than the USB-500mA: Arduino Forum )