I think optional stays are worth it in a frameless pack when paired with a folded up pad, especially when you breech the upper limits of a frameless pack, like at the start of a multiday trip loaded up with food and water.
My Ohm, not a direct comparison I know, carries well in the 20-25lb range with the addition of the inverted U stay, it helps keep the torso from collapsing severely like others mentioned. Pair a folded up foam pad inside the pack with a stay and you have yourself additional rigidity which helps tremendously, assuming the pack fit is proper. If you're comfortably carrying around 20lbs in your frameless pack I think it is safe to say that the addition of a stay can wncrease your load by 5lbs. What a stay doesn't help diminish is shoulder fatigue and discomfort, so make sure your arms and pack are up for heavier loads.