Paul, I believe the reason why people don't tend towards multi/poly is for the reason you mentioned. Multifilaments are not nearly as resilient as gut or poly. Elasticity, however, is not the same thing. Elasticity is the ability for the material to return to its original shape after deformation (think about loading up a rubber band to shoot at someone). Resilience, however, is the ability for a material to absorb energy while being deformed and then return the energy (two rubber bands stretched equally far, the more resilient one will fly further). The amount of resilience (technically called the modulus of resilience) therefore is the amount of energy that can be absorbed before something is permanently deformed. When we want good spin, we want BOTH elasticity and resiliency, but if we have to choose, then it should be resiliency. Why? Because if a material can deform an enormous amount, but it does not absorb and return a lot of energy, then that deformation isn't working for us.
That's what we've got when we're talking about a multi main. Multis are incredibly elastic, but since by construction they are not resilient, they do not return a lot of the vertical deflection (which as you know is now considered an integral component of spin) back into the ball. They tend to reach their limit simply by by being deflected laterally by the incoming ball. If you add in a poly cross, there is minimal sliding friction, so the strings become deformed vertically more easily to the point that you end up shortening their life. That's why people rarely use multi/poly for spin and report lower durability.
Poly, meanwhile, is not terribly elastic. In fact, it's not meant to be. What it is, however, is incredibly resilient so that when the strings are deflected, they load up energy and then are able to return it without exceeding their limit. Death occurs when the string surpasses its elastic limit permanently.
Gut is quite resilient, and it is incredibly elastic. By giving the gut a low friction surface, the gut can make the most of its elasticity as it does not have to overcome as much inter-string friction. Using a multifilament in its place, the string just doesn't return as much incoming energy into the ball as natural gut. Using a poly in the mains instead lets you reap the benefits of having a lot of resilience although with less elasticity. The multi in the crosses gives the string bed some more elasticity in the lateral direction, and this manifests itself in the form of power and comfort/damping.
I guess in short, if you hit flat, then multi/poly shouldn't make much difference vs. gut/poly in terms of spin. If you hit with a lot of spin, then the multi makes little sense in the mains with poly in the crosses.