My point of view from reading both Mo's article and Mike Riggin's article in Bowlers' Journal is that spin rate is a way of actually measuring a factor that Bowlign This month has been referring to for years as the "Torque" of a ball.
It is basically a way to measure the "flippibility" of a ball. Polished and pearls have, by their inherent nature, a high degree of skid/flip-i-ness (pardon the made-up word); dull balls have less flip, mostly because they grab the lanes so early.
However, in Mo's article and method he stabilizes some factors in order to reduce variables involved: namingthe surface of th eball, which he sets to 1500 grit snaded, and the rev rate of the machine, 415, to what he consideres th erev rat eof a *GOOD* bowler, and the surface of the device that contacts and actually spins the ball - a hard urethane wheel.
Keep in mind that, as Phil, indicates, this is just one factor involved in determining the utility of a ball for a bowler.