We pick our most old, used-looking beach towel and bring that with us. I use it to cushion my chair at the pool, as our beach towels are longer than the towels WDW provides and I spend more time relaxing & reading in the lounge chair than swimming at the pool. At the end of the trip, we throw the towel away instead of packing it to bring home. Dh and the kids will use the WDW towels for swimming.
It's nice to have that colorful towel to mark our chair. If I get up and take a quick dip in the pool, then everyone still knows at a glance where our chair is.