Skid marks on newly re-upholstered furniture is not something I'm willing to chance. I sit on a towel in my home office and any other room in our home and outside. If you're too lazy to carry a towel, use a piece of fabric and tie it around your waist. At times when I'm doing yardwork I do since I don't like sitting on the ground w/o anything under me nor on the wood benches near our hot tub...I don't like splinters.
Oops, my bad, one thing I forgot to specify: The setting going through my mind the whole time was outdoors. I wasn't thinking about indoor furniture. I never thought to clarify that. Now that I think about it, I do agree with you, Cheri_Donna, that sitting on indoor furniture in any case should involve a towel/cloth covering. I also should specify this: it's generally a good idea that guests bring towels anyway, but I would try to make certain that some kind of cloth covering is available in the house/building and on the furniture, prepared in advance for when guests are about to arrive. I'd be sure, of course, that the towels/coverings get washed to reuse for the next day of guests.
Also share thoughts as I'll be sharing mine here (and this is for indoor stuff): what does everyone think about towels/coverings in fixed position on seats, no one moving them around? I personally find it most convenient that the towels stay put while guests are free to switch what random seats they choose. That's another thing I'd leave to "guest vote." What do you others think? Is it important that each guest sit each time on the same towel, or is it OK if the guests switch around but the towels stay put?
As for outdoor stuff on hard, stable surfaces -- in a landscape in which stones/big rocks serve as natural "benches" for seating, where guests don't have to worry about splinters or the ground's sharp objects, or on a concrete surface at the edge of a pool or hot tub -- mostly onthose surfaces Isee towels as entirely optional. I can't imagine a worry of skid marks on those surfaces. As for lawn chairs, I haven't quite decided about those. I'm tempted to say that skid marks are fairly easy to wash and rinse out of lawn chairs and tanning recliners, but don't hold me to it yet.