Reading is more comfortable when the text is not sized to the entire screen (cite below, with more references). Also, witness the number of window managers that offer ways to facilitate half-window views (i.e., XMonad, Cinch or SizeUp, Windows 7 window snapping, even Ctrl-X 3 in Emacs, so on and so forth).

http://psychology.wichita.edu/surl/usabilitynews/72/LineLeng...

Where is text sized across the entire screen? There is text across the screen in the linked blog, but it's broken into distinct blocks.

I normally don't browse side-by-side, but it's fantastic for translation from scanned originals. (My two bits.) Way easier nowadays.

OK, let me ask some questions to calibrate your set up and see where you're coming from.

1. What is the width in inches of your display and how many pixels.

2. How many inches from your eyes to the screen in a normal working position?

3. Is your browser set to full screen width right now?

4. How many words per line are there across the screen in this page of hacker news on your set up?

How does this answer the question? You said it was common. I asked if it was really that common. What is your source? It's an important question to ask if you're designing something because it determines how much work needs to be done.

It's why I've considered adding j/k support to my blog. It wouldn't be worth the effort for most things other than a tech blog.