Polyester also stretches less than nylon, which is desirable for a fly or tarp. With silicone rubber impregnation as a coating, the coating adds to the strength of the fabric. Urethane coatings on nylon actually reduce the tear strength of an uncoated fabric, requiring heavier base fabric weights for a particular finished strength.
Personally I prefer polyester for windshirts, and have seen a few Goretex garments that use polyester. This makes sense as polyester fabrics with good dwr treatments hold up well and shed water remarkably well. I’d like to find a good dwr treated 1.1oz breathable polyester such as what Marmot used in their Chinook windshirt, but haven’t found any through the usual cottage industry outlets.
I think the outdoor industry prevalence for using nylon has to do with its traditional availability after WWII when fabric mills geared up for all manner of military and civilian uses for the fiber. Nylon is cheap, common, and available in many grades, forms, and colors. While it will always be stronger than nylon, for most uses a suitable polyester could be substituted just fine. Sails have been made of dacron polyester for decades, and a sail sees considerable windloading. For a ground hugging shelter, lightweight polyester fabrics as used in the SpinTarp perform amazingly. They provide less durability margin for accidents like falling branches however.
