It is a good point. More belt elasticity promotes more creep as the drive pulley surface transfers belt tension drop into driving torque. Creep has three zones of motion: reverse active, active, and passive. Reverse active is where the relative belt motion is counter to pulley motion and does not introduce torque, active is where the belt does introduce torque as the belt elongation drops in small distinct stages (small slip-stick stages observable as chatter), and the passive zone is where creep is constant more akin to slip that moves at a constant velocity slower than the pulley surface.

Note there is a difference between relative motion zones, and driven and non-driven motion of belt and pulley. See "Zeddes" Ph.D. dissertation.

Creep motion differs from large scale motion such a "slippage". Certainly, nylon will induce more creep than steel cord. I think this is less relevant as belt tensions become larger we migrate from polymers to steel or we pile on plies.

Maybe, the question is: will a more compliant belt induce more slippage? Will a compliant belt induce more lagging wear? Will a compliant belt become less stable at higher drive torque or does not obey the e^(theta xf) equation?

Generally speaking, a polymer belt construction demands a larger pulley diameter, at the same strength rating. Since the belt-to-pulley driving surface contact friction is pressure dependent and diameter dependent, the more compliant material somewhat self compensates.

Side notes:

http://www.euclidres.com/apps/pulley/slip.html

This problem becomes more complex when there are multiple drive pulleys. ■