Text: It is easier to read pages on top each other than side-by-side
There are these weird things called books that might dispute that assertion
Books are limited by physical constraits which require a continuous text to be divided into pages. Computers are not so restricted and provide flexible layout and continuous scrolling. In this situation a portrait screen is more appopriate because the eye finds it difficult to scan very long lines of text (which is why books are usually portrait and why wide pages are often divided into multiple columns).
The good news is that computers give you the flexibility to choose whichever option you prefer.
Command-line: You can see more lines of output
Reading log files with wrapped lines or writing long, one line scripts is far better on a landscape-oriented widescreen monitor. I'm not sure what's wrong with shift+Page Up if you need to see more lines of output.
My personal view is that if you are generating very long lines of output or typing very long command lines, you are doing something wrong.
I get 150 columns in portrait mode which is enough for me, YMMV.