The carpet stretchs more as there is more movement of the carpet. resulting in it losing its tension which results in extra wear. Any contract underlay is firm. Cloud 9 also do a contract underlay for heavy wear area which is more dense and thinner.
Sorry to revive this old thread. I am doing some research on underlay for my bedroom.
Does your comment apply only to stairs or to rooms as well? Im thinking of going with the 11mm thick Treadaire Dreamwalk or Cloud9.