[ahecht]

I don't want to get too much into motor theory, as it's been a while since I've done this stuff, but basically the job of the yoke is to provide a path for the magnetic flux. As I remember it, having a steel path between the magnets instead of air increases the flux density, and therefore the efficiency of the motor (someone correct me if I've got this backwards or plain wrong). The more steel the better, up to a point when things become saturated.