It all comes down to distributing pressure/stress. Let's say you have a stinger on your bumper, and you tip forward placing the weight of your rig (and our 4-door JKU's are NOT feather-weights) on your stinger/bumper combo..., well when this happens the speed of the roll/tip coupled with the weight of the vehicle and the decline of the roll/tip, will give you the force or pressure that you'll place on your bumper/stinger combo.
Although Grade 8 bolts are extremely strong, especially when mounted through the bumper mount at the frame (like I believe the EVO, and perhaps others do), those bolts will shear at some point. There is a much higher likelihood of shearing the bolts with a bolt-on stinger at the bumper/frame mounts, because 100% of the load or force, will be placed on the stinger/bumper bolts FIRST, and then if it makes it past the bolts, it will transfer the load or force onto the next thing in line.
If you have a welded bumper/stinger combo, then you reallocate the force or load of the vehicle across all of the welded points on your stinger, and if done correctly, to your entire bumper, which then steps down the load or force to your bumper, and then the frame mounted bolts.
My preference is the welded option, which is why I went the route I did, but as they say..., "Opinions are like..."