My 2 cents on this issue is, I belive that every load should be rigged accordingly. that being said if i were to load my offroad vehicle with FLEXY suspension to a flat bed i would use chain and binders on each end ATTACHED TO THE AXLES.
Then you don't understand the purpose of securing at the chassis, ESPECIALLY with a flexy rig.
A vehicle or trailer is basically an overgrown bobblehead doll. The "head" of the vehicle is the heavy part. The bottom (tires, axles, etc) is relatively light. Securing at the axles, the heavy "head" is allowed to bounce around.
Look at it from the ground up: You have trailer wheels and axle, trailer suspension, trailer body, jeep wheels and axle, jeep suspension, jeep body. Securing from the chassis, you secure the entire jeep to the trailer body, creating a single live-load on a single suspension system - the trailer's. Securing at the axles, you have a suspended load on top of a suspended load. The suspension systems are allowed to interact. This can greatly increase the possibility and severity of sway. Ever rocked back and forth in a car? A hundred pounds of live-load can throw a small vehicle around pretty good; care to try the same thing with a couple thousand?
Sure, you CAN secure only to the axles - with a heavy trailer, a heavy tow vehicle, and a decent road, you're probably not going to have any issues. But why take the chance that your jeep's load will be shifting on the trailer in the wrong way at the wrong time? Like when you're maneuvering to avoid killing a family of 4 stopped partway in your lane?
Am I saying that securing to the axles is wrong? Of course not - the axles are a very strong tie-down point. I use chains and binders at the axles to make sure vehicles don't separate. I'm saying that one needs to consider the *possible* effects of the live-load, and take appropriate steps to mitigate these effects. Is it really that much extra effort to rig two straps to lockout the suspension?