I lived on my sailboat for 22 years working for the University of California. Every new position was a new marina! I sewed all my sails and bags and found that the really stiff Dacron stuff is really tough to handle, especially the storm jibs and some of the mains.
Spinnakers on the other hand, are typically ripstop nylon and they are generally really large. (my 37 foot sailboat had a spinnaker that took 56 yards of 45 inch material to build. It was huge) If you can score a spinnaker then you will have nylon for many projects.
As others have pointed out, the scrap material is usable for lots and lots of general project. Sometimes it helps when sewing on the heavier Dacron to spray your thread with silicone. It seems to slide a bit better, and make sure to replace your needles fairly often, they dull really quick on Dacron.