Dedicated cab is the key here. I thought more in desktop terms, as that's all I have. Yes, nobody says that Pacman is not 3:4 here, but there are quite a few games that are not 3:4, so having widescreen monitor is a big plus. Besides, if you go to a store that has them on display you will notice that they are not all the same shape, i.e. same aspect ratio. Some are longer than others, so all of them are not built to the 6:9 ratio, just like with the movies.
Anyway, having option to rotate desktop widescreen lcd is great and some games like Darius, Ninja Warriors, etc., Playchoice and Mega-Tech systems would display great in a widescreen. I think even Lunar Lander could be considered widescreen, if you don't use any overlay on the screen. Hmm, I wonder if cropping overlay on a widescreen monitor could give benefit by filling empty space and not shrinking the game screen as much.