I don't think they are structurally necessary for a camper unhooked, kinda like op asked. But i think they could have benifits.
I dont have one, but i want to try one this season. With a 42', 4 slide camper with at least 3 kids bouncing around, and me and the wife rocking the trailer, im going all out this season.
Yep, we have jt's, and rear stabs, and use 4 chocks, but i want to add a tripod, 2 middle stab jacks and x chocks. Being sessonal, all this shouldn't be a big hastle and hopefully will make rock solid.
Op.... i think every camper, campers in it and situation will be different. Not necessarily, necessary structurally, but maybe for stability or comfort.
Good luck on your decision!