If you take a fall on a harness, leg straps take majority of the load in most cases. Without leg straps the waist belt would stop at your ribs or not at all.
What saves saddles from being completely dangerous is the bottom edge of the panel sits under your butt so even if the leg straps hold the bottom edge in contact and low enough, it's likely your weight will end up on the bottom edge of the panel.
I notice while onesticking and hanging to lift the stick, sometimes if I straddle a big tree my legs start to push the panel back and I start to creep over the edge of the panel. Very uncomfortable feeling without functioning leg straps. I'm planning to gut a rock climbing harness and apply the leg straps on the harness, just need to find some with quiet enough material. As a bonus this gives me a belay loop for more efficient SRT climbing.