my tarps in particular (don't know of anybody else who does it this way) have edge binding on the ridge seam, that edge binding is very strong (i've stood on a strand of it hung like a jumprope and bounced without it breaking, i'm 160#), it's all the additional support you'd ever need. the main problam i have with a seperate ridge cord and prussics is that if you pitch the tarp tight enough the pussics will slip, aprox 40-50# which isn't that much.
i always tell people using my tarps that over is better than under. i've heard of one instance of someone using a seperate ridge cord "under", and for whatever reason the ridgeline seam of the tarp was slightly looser than the cord under it (because of the prussics i assume), so when the wind blew there was enough slack that the seperate ridgeline rode higher than the ridge seam (while the wind was pushing on things). basically the seperate ridge cord and tarp ridge seam were not lined up, so the seam was not at the apex of the ridge like it's supposed to be. this allowed water run "under" the edge binding and to come through the RL seam since it was not the highest point. that would't happen if the RL seam was taking all the tension like it was designed to do. i've set my tarps up tighter than anyone ever will in the field, they don't need any extra support and pitch best when the seam is under plenty of tension.
i don't mean you need to crank down on the RL guylines, but when you tighten the ground corners, it should fully tighten up the ridge seam, and having a seperate ridge cord (especially under) can prevent that.
keep in mind this is just what i've experienced using "my" tarps with the built-in ridge support