If your lines stretch, you have also potentially lost power to the sails, and speed, hence why one would want to use a less stretchy line for sheets, along with halyards. Not to mention the sail cloth itself, hence why racers use carbon and kevlar with mylar too in sails vs dacron these days. Many cruisers are using this higher tech, less stretchy material, so they do not have to spend a lot of time readjusting things for stretch, along with getting more out of the boat for a given weather conditon too.
Some of this stretch could cause more heeling without adjusting thing constantly too, as you will not have the sails at the proper setting for any length of time.
Many reasons to go with less stretchy lines and cloths. It comes down to how much you want to spend, how much improvement you want etc. Plus's and minus's to both!
She drives me boat,
I drives me dinghy!