That's a very good idea, which I hadn't thought of! I'll make some next time I scrap a tube. Obviously, bands can be made by cutting across the width of a tube (MTB tubes would be good for longer ones made that way), but bands of arbitrary length could be made by cutting strips along the length of a tube and then joining the ends with a reef knot.
Mind you, our local postal workers drop so many used rubber bands on the streets round here that I don't really need more!