Seriously, most of todays screens are so big that you can fit 2 pages side-by-side, which is a lot more convenient than one page at a time in portrait mode. Ditto for individual windows. Rotating them into portrait mode will cause neck strain as you have to tilt your head back to properly see the top.
Have you tried it? I have two 24" monitors, one portrait, one landscape. I use it for writing code, where having more vertical space is really helpful. My eyes are even with the center of the portrait monitor, and I don't get neck strain at all. Things on the very top and very bottom of the screen aren't quite as comfortable to look at, but general those are things that only require occasional glances (IDE menus, status bar, etc). Overall, I find it incredibly useful, and quite comfortable.
Sure, it may not be for everyone, but why state negative assumptions about something that you haven't tried?