The reason I can get more on a 16:9 screen is that I tend to size windows with their heights proportional to screen height with and widths proportional to the vertical size of the window (I very rarely run anything full screen). This means that for me extra width gives more windows simultaneously – while I can get more on a bigger 4:3 screen it usually ends up with a lot of the extra real estate going on larger versions of the same things as I would have had before. Obviously, that’s just what I find comfortable and won’t apply to everyone – it depends on the user.

IIRC the tablet PCs I’ve seen have been widescreen and supported 9:16 mode but I never paid that much attention to them due to a lack of free software support.