Spectra is intended for uses where low-stretch is important. I use it for flying stunt kites. Stretchy lines reduces the responsiveness of the kite. But that's really thin line I use for that, 50lb to 150lb test. My buddies who kite-surf use heavier spectra. It's also used for sailboat racing, spinnakers also need delicate handling, and low stretch keeps them from needing re-trimming in gusts.

I like a little stretch in my boot laces, so spectra would be the last thing I would use.