All those points are valid. However the scaling with single image is meant for fluid width only, for people who want the header to be fluid width to match their tables, as explained in the thread about it. Until now this could not be done without using multiple images to accomplish the result. Problem with that tried and true technique is, the images stack on top of one another on small screens. With this little trick, it doesn't.
I took the liberty of putting this header image on one of my styles that looks very similar to Avenger style as a example, here:
Technique style on my board
If you shrink the width of your browser while viewing this, you'll see what I am talking about. header and tables are all 100% fluid width and there is no stacking or wrapping possible.
It's just another design tool for the toolbox, it's not meant for everything we do. Problem is, I still don't know if it works in v4 or how to accomplish it there.
(By the way, that TOTAL KB load of that entire page is only 110KB. Entire page is smaller than your one PNG header image.)