What's the purpose of soft pedaling?
By this do you mean pedalling when unnecessary (and without increasing the bike's speed), for example, on descents?

I will do this on long descents just to keep my legs turning over and to prevent them getting too cold. But I will mix it with coasting. 

There is a theory that keeping the legs moving will keep your blood flowing and help pump lactic acid out of the muscles. I can go along with this as I've felt the benefit, or at least I think I have.

I've also heard it argued that it's advantageous as regards making your technique smoother, but I don't think that's particularly proven. But no personal experience, I've never really concentrated on my technique to that extent.

Further, you can talk to racing cyclists who will often say that soft pedalling is preferred when riding in a peloton, purely because coasting implies to other people that you're about to hit the brakes. Again, no personal experience. I know some of the regulars on here are either racers or ex-racers, maybe they can confirm?

In terms of speed (or power) surely it makes no difference, purely by the definition of "soft pedalling"?