> But whenever I've done a spontaneous unplanned dismount at 20mph I didn't find that knowing how to fall helped me much.

What was the circumstances (type of road and type of bike)? Have you touched the ground with any other parts of body except of palms, elbows and knees? If yes then consider to keep learning how to fall because your falling skill is not that proficient. If not then the helmet was not that useful.