It's a safety thing.....In that case, it's a cheap precaution to what could happen.
I haven't had any problems with using a shackle for the rear, but I can see how it could be bad. I have only used it in the driveway, and never leave it resting on just the jack. The first thing I do is put jack stands underneath the truck. To bad this is not a viable option in teh field and on the trail..
Cliff