Your welds actually look really good, except for the stop/starts around the top and bottoms, but that's hard to avoid without starting/stopping, which is difficult for a home fabricator. Next time maybe use a very small table/clamp that you can circle around as you weld. But of course that's a lot of work to make them look pretty.
I also wouldn't have welded them completely together, just at the top and bottom. But I'm a minimalist.
I agree with you as for their necessity. BMW spends a great deal of R/D for even something as simple as these. And then the material, and time, cost to include them on every car? The savings would be fairly significant if they were not necessary. But they are necessary to enable the car to meet its goals.
BMW dropped the twin turbo for the single turbo N55 was done for one, and one reason only, and that's to equal performance at a lower cost. If there are pennies to save, they will cut those corners, no doubt about it.