I should have mentioned this earlier but I forgot, you did not say what kind of hub you are using. Some hubs (SP for one) are not grounded to the frame, but Shimano is grounded to the frame.

And some lights are grounded to the frame, some are not. If you are using a Shimano hub with a light that is grounded to the frame, you likely will find that you have to make sure you do not accidentally get your wires crossed, as there will be a right way and a wrong way to wire the light to the hub. With only two wires, there are two choices and if it works, you got them right. And if the taillight is grounded to the frame, that gives one more opportunity for things to get complicated.