Spectra is a very low stretch fiber. It's useful in situations where low stretch is desired. For fishing, you can feel the fishes nibbling on your bait a lot better. For stunt kite flying, you get better control. For sail handling, you don't have to re-trim the sails as much when the wind strength changes.
However, it doesn't take knots very well. When I use it for my stunt kites, I put a sleeve and a loop on the end to tie it to the kite. I'm sure fisherman do something similar.
Paracord is much more versatile. You can take it apart and use the inner strands for other things. And for most things, a bit of stretch helps make it stronger by cushioning loads when they hit the end. And you can tie knots in it without reducing the strength.
Note while non-stretch stuff certainly won't take knots as well as something that stretches, the braided Spectra (reference the link I provided, above) takes knots much better than the solid core stuff. I really haven't had a problem in that respect with this stuff.
I also think that part of the knot holding issue is how tighly the core is bundled. The Technora stuff I have is packed SUPER tight and I think that is part of the problem. The braided Spectra cord is not packed tightly.
I find the low melting point a good thing because I like to melt the ends to prevent fraying. I have some 3mmTtechnora cord and that stuff doesn't melt and is a real pain the rear to keep it from unraveling.
Spectra is also very light, doesn't absorb water and floats.
The braided cord can be unbraided if you want smaller strands.
I think the cord I linked to is just as flexible as paracord and is rated for something like 1200 lbs. Even if you reduce that by 60% you are still looking at something like 480 lbs.
I'm not familiar with the Dacron line, but I plan on checking it out.
-john