The relative stretchiness of dacron lines I believe is fairly well known, and important.
I think there is something about the friction between dacron lines and slider grommets that is "just right".
Ever since I've had my Pilot relined with Dacron, it has been "just right". I don't know if it is as simple as coefficient of friction or if it is a more complicated relationship with other properties, but I think dacron lines make it easy to get nice openings. I think that slippery line types, such as spectra make it more uncertain because of their different frictional or other properties. I have no experience at all with other types of lines, perhaps they also provide that just right interaction with the slider grommets.
I think more of us that jump larger canopies should consider dacron when a reline is needed. Dacron's ability to not get snagged on velcro or other such similar natural or synthetic materials is also really nice. I always felt like I was destroying my lines when they would get snagged on some velcro from my jumpsuit wrist cuffs, or some vegetation that would get snagged...
Anyway, I think early canopy designers benefited greatly from the good characteristics of dacron rubbing against grommets, and we would be grateful if other line types had that just right relationship, even if they weren't stretchy.
Just a random ramblin, what do you think?