Modern road race, rally, and drag race cars, that are based on any new unibody chassis will see significant torsional strength improvements by seam welding them. After all, all newer cars are built from an amazing number of sheetmetal parts that are spot welded, or spot welded and chemically bonded, together to form a complete body structure. And while these methods are designed to be strong and safe in an on-road environment, that is not the same thing as a race environment. Especially when you are talking about adding much more stress from suspension and horsepower changes.
So lets say you are taking a newer car and stripping it down to just the shell in order to install a cage, etc. No matter how if you remove every single part that can unbolt, you are still going to have lots of seam sealer, some bonding materials, etc that is at every joint where to separate pieces of metal come together. And while those materials are fine for the street, repeated impacts, stresses, and load can weaken those joint. Perhaps not to the point of failure, but certainly to the point of causing unwanted flex within the chassis. If the chassis is flexing, instead of the suspension, no amount of suspension tuning will get your race car working perfectly.
In this video you’ll see the why and how on seam welding a chassis. In this case, it is a rally car, with a full cage, but welding all the seams still provides major structural improvements. Does that mean every car needs it done? No, but if you are going to take one apart to this level anyway, there seems to be no reason not to. Pun intended.