If you check the California vehicular code, they require a front light that's bright enough to meet a particular specification. The code is silent about a rear light, just requiring a reflector in back.
You might think that this is backwards. And, given by the number of folks I see at night without a front light, a lot of people think like this.
Clearly a headlight of sufficient power is required on most any vehicle because, in the absence of other sources of light, you need it to move forwards without falling off a cliff or something.
On the other hand, if a car is coming at you from behind, they've also got their headlights on, will be fairly likely to illuminate the CSPC angled reflector on the back of the bike, and will be fairly likely to illuminate the cyclist in the absence of a reflector. Very few drivers will continue driving with failed headlights. Again, this also works for cyclists as long as the cyclist has a headlight.
Furthermore, consider closing velocity. If a car is approaching from behind, the closing velocity is the difference between your speeds. If a car is approaching from the front, the closing velocity is the sum of your speeds. Therefore, if you can only have one light, in general a forward-facing one is going to help more.
I've almost hit other cyclists when they were bike ninja riding salmon for this reason. My bike headlight helps me see, especially under traffic lights, but it's not nearly as bright as a car headlight and not nearly bright enough to illuminate a reflector-less bike from a decent distance.
It's still a good idea to have a rear light, of course. If you've got a car with the passenger-side headlight out, the light will hit the reflector at exactly the wrong angle to have a bright return bounce. And, while cateye reflectors on the street and lightposts are great in the fog, fog does reduce the effectiveness of reflectors because it's scattering the light going to the reflectors as well as the return bounce.
Overall, I tend to think they could enhance the legal requirements, but most folks either don't bother with any lighting hardware at all because they are either too broke or think they are invincible... or they understand what's going on and exceed the legal requirements by a significant margin.