I have eaten that way since I was a child. It seemed, as you said, both more efficient and quieter. My mother tried for years to get me to eat "properly". Eventually, I looked it up in an etiquette book, and they said either way was acceptable. So at least we stopped fighting about it. The only time to use zig-zag, as far as I can tell, is when the people around you are slow eaters, and you need to slow your eating speed to closer match theirs.