You absolutely should plate the subframe, because if you have a major impact it could go right into the subframe. You only need a plate about 4 inches larger that the cross tube. It'll help carry the load incase of impact.
I did not do mine, as my sliders should up in ups as I was packing for West Coast Camp Commander #1. I forgot to order the plates and didn't want to wait. Shame on me, but they have done exceptionally well.
I have seen 1 brand of XK rocksliders fail on the trail. It uses nutserts to attach to the rocker panel. It happened on p@55w0rd's XK at Badlands 2 years ago. The rocker took a hit, up went the slider into the bottom of his fender and door. This company is out of the making sliders business. I believe the cause of failure was the small nutserts and poor design combined.
