Those reverting to single security are only doing so on "little" gassers. Not many 50cc and up going that route.

You are correct in stating the center nut provides equal force over the face of a prop washer, but no more or less so than a mulit bolt. With a multi bolt those forces are moved outward a small amount. Better-worse? Don't know. In either type over torquing really screws up a prop. Pull most any wood prop at your local field and look at how deep the reaf hub indentations are. You will find most props are grossely over torqued unless a torque wrench was used.