Stretched aspect ratios annoy me as well.
Systems respond differently to the ratio information, I guess. On my other system when I'm watching old TV-series, they are 4:3, and only the commercials come wide-screen. On the other system, if I change the aspect ratio to 4:3, the commercials come in 16:9, and after the commercials the program continues in 16:9. I hate it. The stretching also decreases the picture quality noticeably.