I would contend that a) the space difference between a usb and a simple two prong extension cord is negligible in most luggage for the same length (I always travel with a couple of cameras and a laptop, there's always room for cords.) and b) you can find the two prong extension cords pretty much anywhere you go and at a cheaper price than a usb cord (Just a quick look turned up plenty at $2 but I bet they can be found for cheaper than that.) which means in a pinch, you can buy one and leave it behind if need be. Also, if you ask at a hotel desk, I'm betting they can get their hands on an extension cord a lot easier than a usb 3 cord. That is if they even know that there are different types of usb cords.