Except it's well-known that humans read more quickly if lines of text are kept short (around 6" or so). The height of a page doesn't significantly affect reading speed (ignoring scrolling). So it makes more sense, for reading, to have a narrow, tall monitor. (This is why LaTeX defaults to wide margins BTW, and I'm pretty sure this is why Kindles etc. are portrait-oriented.)

Even on a narrow monitor, all websites since suck.com use snake text these days, so the width of your text columns is likely to be unaffected. The only exception left seems to email software created by Unix folk who prefer all text to be forced to 80 characters wide monospace.

Using a narrow monitor will only lessen the amount you scroll.

Monitors are not only used for reading, for watching movies and playing games 16:9 is better than 4:3.