As posted above, at the Deluxe and Moderate resorts, pool towels are provided. They are large bath size (not sheet size). I've always found them large enough, but there is nothing stopping you from using two, if need be. I think this is preferable. I have found that nothing ruins a female swimsuit faster than touching it to concrete (don't get me started on the mysogeny of female swimsuit industry...). Male suits are fine, but lycra is evil. So I think being able to use multiple towels helpful. If I plan on sitting on the side of the pool/hot tub, I use one for that, and if it gets wet - I can just grab another one if need be. I also like not dealing with so many wet towels back at the room - drop them in the bin and be done with it.
Few people use their own.
The Disney ones are also bright white and very clean.