I'm not saying your wrong but I don't agree the weld is the weakest point. I was in the pipefitters union as a welder for years. We were taught if you use the right rod (stick welding) that has a higher tensile strength rating than the base metal the weld is the strongest point.
That is true but only happens if you know how to weld.
And that is rarely the case in DIY welding.
To properly stick weld something as thin as a frame is a pretty rare talent.
What usually happens is they grab a rod out of a can thats been open and in the unheated garage for 5 years and soggy as hell. Aint no amateurs using a rod oven.
And they dont notice the flux popping off of the rod and landing in the weld puddle in chunks.
And they dont prep the joint or preheat it or multi pass/ backgouge etc..
But they will blow half a can into a 4 inch weld.
So you are right, it may not be the weld itself that fails.
But the undercut base metal that got heated into taffy right next to that weld is weak as hell.
So the weld doesnt necessarily fail, the heat affected zone adjacent to it fails.
MIG has its own set of problems including bad prep lack of penetration not understanding the difference between spray transfer and globular etc..
Wrong gas, no gas, welding in the wind while the shielding blows away and on and on..
All of this leads to porosity and inclusions and surface welds with no penetration and welds that look like a porcupine from all the wire sticking out.
Im sure youve seen it.
Boxing the frame not only transfers the loads, it also adds redundancy to the structure because it has more welds spread over a greater distance.
This is especially important if those welds are sketchy to begin with.
And dont even get me started on people who weld frames with a harbor freight flux welder and think they are MIG welding