The Revised Code of Washignton 46.61.780 ("Lamps and Other Equipment on Bicycles") states that bicycles used at night "shall be equipped with a lamp on the front which shall emit a white light visible from a distance of at least five hundred feet ... and with a red reflector on the rear.... A lamp emitting a red light ... to the rear may be used in addition to the red reflector." In effect this means that red lights MAY be used, but that red reflectors MUST be used (a 1998 amendment specifically allowed the use of blinking rear lights).Cyclists should comply with the law. It enhances their safety, enforcement ("on" cyclists) will only become more common in the future (less than zero enforcement is not possible), plus, in the event of an accident, such compliance may help achieve favorable results in court or with insurance companies.
Mounting a red reflector on the back of a bike is a minor inconvenience (most bike shops will give them away for free). Perhaps more importantly, a reflector may prove effective when rear lights fail — and it does happen. Because rear lights are not in one's field of view while on the bike, the realization that a malfunction has occurred mid-trip usually only occurs after one's destination has been reached.
Lights are "active" safety devices, relying on batteries, wires, solder joints, switches and bulbs to function properly (and they rely on the cyclist to acitvate them). They are more complicated and thus more prone to failure than solid state, "passive" technologies like reflectors — when any of these parts of an active system fail, the system as a whole provides no safety benefit. The passive reflector, in comparison will remain effective indefinitely because it only relies on incoming light to function properly. Admittedly, reliance on incoming light is a significant weakness of reflectors, since visibility is desirable regardless of whether an overtaking road user (motorist OR another cyclist) has, and is using, properly functioning headlights. If they are not, the reflector may not offer any safety benefit.
Use both a red reflector and a red light on the rear of a bicycle. This approach complies with state law, makes a cyclist more visible and compensates for the shortcomings of either system individually. This is not meant to suggest that a rear light and reflector is ALL of the visibility gear that should be relied upon.