New to the Pi, so new in fact that my very first (a B+) hasn't even arrived in the mail yet...
I know that "backfeeding" or "backpowering" refers to powering the Raspberry Pi via a powered USB hub connected to the Pi through the latter's USB port (rather than the micro-USB power socket). I've seen lots of discussion about how to avoid it, ranging from "avoid these hubs" to even solutions involving custom USB cables that exclude the +5V wire.
But why is this such a big deal?
A USB hub runs at the same +5V that the Pi itself runs on. And the output side of the Pi's power supply is connected directly to the USB ports' +5V rail (at least that's what I've gathered from the simplified schematics I've looked at). So what does it matter that the +5V comes from the USB hub instead of the Pi's own power supply?