I'm currently slipping into my asbestos undies because I can hear the flame throwers lighting up right now but I take a very different tack on side reflectors...
They are mostly useless.
That said, I do have them on my bikes but only because it is required by my state's traffic laws...as it is in all other US states. But, in terms of effectiveness, they are very ineffective. The angle of an automobile light that will illuminate them is too shallow.
Even a CPSC study agrees. I came to my conclusions before reading the study but they reach much the same conclusion which is backed up by data. From their conclusion (see page 18 of the report which is on page 21 on the link), there is no side treatment that could be detected in less than 200 feet by an approaching vehicle. Most of the side reflectors had closer to 150 feet of detection. To give some indication of what that means, at 30mph, a car covers about 150 feet in 3 seconds (44 feet/second).
And the time of detection is greatly reduced if the sight lines aren't clear. Many of the streets that I commute on at night have fences, plantings, or other landscaping obstructions right up to the corner. The clear line of sight for a car is about the size of a sidewalk and a parking strip. The total distance is roughly 12 feet before the car's lights can illuminate any kind of side reflector.
The CPSC study was done in the late 90s. Things haven't improved any since then either. If anything, the angle of the light from a car's headlamps is more constrained given the more highly focused beams that have become prevalent today. With the lower angle of light, the beam from a headlamp doesn't illuminate as high anymore so any obstruction at a corner is blocking even more light than it did 20 years ago.
Active lighting might be better but, again, the distance at which they can be seen is rather small and would also be impacted by obstructions at intersections. A bright front light and/or one that can be moved to aim at the drivers (i.e. a helmet light) is more effective than any kind of side reflector or side light.