I don't think it is harming the quilts in most cases...probably have a sheet or something under the quilt to protect it if in a funny location...
After my Nana died, my grandfather decided to use 2 old quilts that Nana's Aunt had made (treadle machine & hand quilted) to cover the snowblower in the garage and in the trunk of his car around the hand lawnmower that he took to the cemetary. I only found them after seeing the one in the trunk in shreds...grabbed to other one off the snowblower and marched into the house. This has to be 30 years ago...I can still remember the argument with Pop. He wanted to know what he was going to cover the snowblower with??? I told him to use his winter coat!