I've always gone to the frame with straps running to the corners of the trailer at approx 45 degree angles- going on 25 years now, and never lost/loosened or broke a strap. In the enclosed trailer I have to, because the Jeep will end up hitting the ceiling, but I have also found that the TJ ends up moving too much when trailered, and actually induces sway when bound by the axles. I have never worn out a spring from binding the suspension, but I would imagine you would use up the springs quicker by making them flex the whole ride on the trailer. The trailer is designed to carry its rated load, and the suspension is setup for 10K/7K/etc without needing the load to do part of the work to suspend itself.
I personally like the idea of a single supension taking care of the road instead of two seperate setups working independantly, possibly inducing a situation where both suspensions top out at the same time, not to mention the sway and lean issues a coil sprung rig can have when topped out on the its suspension. (Think Jack in The Box)
Michigan- The Only
Great Lakes State South of the Mason-Dixon Line.