Man, I hate these arguments.
Look, if you have two monitors that both have 15" diagonals, and one is a 16x9 ratio and the other a 16x10, the 16x10 will be narrower than the 16x9 but still have a slightly higher area. If you do the trigonometry, you'll find:
Code:
16x9 16x10
width 13.07 12.72
height 7.35 7.95
area 96.14 101.12
More to the point, if you look at common resolutions you'll see 1600x900 [16:9] and 1680x1050 [16:10]; 1920x1080 [16:9] and 1900x1200 [16:10]. Among similar *available* resolutions, even when the 16:9 has higher horizontal resolution, the common 16:10 has more total pixels.
Additionally, except when watching movies full-screen [and letterboxing a 1080p video on a 1900x1200 screen doesn't really make your movie any smaller], people tend to need additional vertical space more than they need additional horizontal space.
You don't need loooooonger lines of text, because that's illegible, but it'd be nice to have to scroll less.
Even if you move your dock to the side or hide it, all those toolbars on your browser and word processor and email editor are taking up vertical rather than horizontal space and leaving you fewer and fewer lines of actual content you can see at once.
And most cameras out there take photos in a 4:3 or 3:2 aspect ratio, so you need even more vertical space and less horizontal space than a 16:10 screen gives you if you want to make the most of your screen when viewing or editing them.