Coefficient of friction is just one important specification of rubber. What about ribbed, flavored, glow-in-the-dark, etc?
Spadout's results are limited in value, as they don't usually list their assumptions, which for this case include:
1. mu is constant for every normal force
2. mu is constant for every relative humidity
3. mu is constant for every degree of surface roughness
4. mu is constant for every type and amount of surface contaminate (chalk, dust, organics (moss, lichen, etc), etc)
5. mu is constant for all phases of a shoe's life (new in box, broken-in, thrashed)
Most of spadout's work seems to be a proof-of-concept for what a testing program might actually look like. The results are valid, but limited by the (usually omitted) assumptions.