The weld in and bolt in do different jobs, with the bolt in kit doing more by itself than the weld in. The bolt it triangulates that corner of the frame to keep it from flexing. It also keeps the bolt holes from tearing out since the mounting bolts go through the brace.
The problem seems to be in that when you stiffen up that corner of the frame, you still have the suspension trying to twist the frame around that area so you can still get some cracking but it's not really from steering forces but from the suspension. This is where the weld in kit comes in, it spreads that stress down the frame rail and strengthens that critical area.