Interesting that your swingout not have a double-shear bracket/brace already. It looks like you have a 2nd-gen Tacoma as best as I can tell from the pic. I thought all the CBI Tacoma bumpers were already set up in double-shear (like the pic below from their website)? But I agree with you, if there wasn't a double-shear bracket there, if you had all the weight of a spare wheel/tire, gas can (or two), and a Hi-Lift, you would probably get a significant amount of deflection when the swingout was opened up.
I never even installed a set screw on mine, and honestly never even thought about it being there to hold the cap in place since the bracket is already securely bolted to the raised portion above the taillight (i.e. it's not going anywhere, and if it is, a set screw isn't going to stop it). I've been meaning to get a set screw to install in that hole just to prevent it from being another place for dirt to enter the spindle/bushing area. I actually use that set screw hole as an oiling port on mine (when I talked to Nathan he recommended 3-in-1 oil for the hinge).