Actually, this advice can apply even if you have one "Uber" light - under the right lighting conditions, you can wash out all visual reference to yourself except for a single blinding red light source. This can make it nearly impossible for a vehicle to properly gauge your distance until they are extremely close to you, when you're illuminated enough to overpower the red light.

Having more than one light source (for example helmet and back of bike) can give the driver a better visual cue for distance as the lights diverge.

On a related note - the rear clip on the Metro is pretty tight. Depending on the light you may have some difficulty reaching the switch without fumbling and/or removing the helmet to do it.