When I said that a regular torch can do what this one does, it's true, but for many this torch will be easier to set and adjust. I've seen the demos of the Cobra torch, and they are impressive, at least in the hands of those that demo.
As for mig and tig - tig was invented in the 30s I believe, developed for welding aluminum parts for aircraft. At the time, chrome moly for lightweight aircraft frames was welded with gas welding. I don't know when mig was invented, but it wasn't widely used in industry until the 50s and 60s, maybe even later.
As for gas welding on a quarter panel - well most shouldn't, but... I attended a metalworking seminar put on by the old customizer Gene Winfield. He, with help from the class, chopped the top on a 41 Chevy in 1-1/2 days. Only one joint was welded with a mig - the rest were done with an O/A torch. It was obvious that Gene was very good with the torch, he uses an ordinary style torch made by Smith. There were several welds across nearly the entire top. He welds 2-3 inches and while the weld is hot, he planishes the weld. Then he welds another 2-3" and planishes, repeating until the joint is complete. This pretty much removes the weld bead and minimizes the distortion. Little to no grinding, the area is metal finished with a hammer and dolly, alternated with filing and sanding. He even welded the A and B pillars with gas, saying he could get better penetration on the double thickness there (and I think he's right on that car).
I've also seen gas welding of aluminum body panels for a race car where similar techniques were used - no way could you do something similar with tig IMO. Secret to gas welding sheet metal is 100% access to the back of the panel as you go. Hard to do that with a trifive quarter panel.