I wish a real engineer would jump in. I can only offer my 'redneck' take on the engineering. My understanding is that a steel spring has no damping at all, and that rubber *can* have some damping effect, because there will be some 'scrubbing' effect as the rubber compresses (and spreads out), and decompresses. It absorbs some of the energy because it's converted to heat. Of course, that means that it can't be 'springing' (by very much) constantly, or the heat will kill it. The good thing is that if it's designed right, it won't need to. (See below.)
Having said all that, I suspect that the damping effect isn't as important as keeping of the resonant frequency of the *system* (regardless of method), outside its normal operating range. Other threads, by real engineers, cover it a lot better than this uneducated redneck ever could. My understanding is that the critical thing is to keep the system out of resonance.
Again, my interpretation; hopefully a real engineer will clarify. If you haven't read it yet, Billsky does a great job here:
Torsional Vibration and Resonance - Basic Theory and Issues
Charlie