https://www.raspberrypi.org/help/faqs/#powerHub indicates that certain USB hubs can "backpower" the the Raspberry Pi by its USB ports. Apart from the fact that it seems to bypass the overvoltage protection circuit which is afforded to the "normal" micro USB power connection, there doesn't seem to be a reason not to do this.
However, this could be specific to previous models (RPi <3) as I read elsewhere that it's not possible to power the RPi 3 via its USB ports. I haven't seen a schematic for the RPi 3, so I cannot confirm. Can anyone help, with either an schematic or a citation as to whether it's possible?