Razor back is not nessesarily a horse with just high withers but prominant withers with a spinal column that sets high, ridge like (ribs that come off low from it). It is often associated with slab sided horses, older horses or horses built like grey hounds. Saddles are usualy hard to fit and often built up pads are used to aid in keeping saddles from riding the "ridge" and friction. Sometimes having a narrow twist and thicker that usual flocking in a well made english saddle will fit these horses more comfortably. They are not fun to fit saddles to though either way you look at it.
Riding them bareback definitely makes the rider become more softer, up thier buttocks and thighs more effectively. I had an older walking horse mare that had a prominant spinal column "ridge" (though not realy bad) and I rode her backback often. I learned relaxation and learned not to ride with my pubic bone so much and also learned to use by buttock muscles more effectively. Severly "razore backed" horses I thwarted riding them bareback and opted for the saddle. Easier on the both of us. :)