80 characters is nice for side by side buffers. Also, lines that are too long horizontally are difficult to scan (especially if they are much longer than the lines around them) and can benefit from some work on vertical layout.
I get the difficult to scan point (although I find it subjective) but the side-by-side argument is optimizing for a comparatively small use case.
> How much time do developers spend on side by side buffers?
In my case: all the time. On my desktop (which has a large display) I often tend to have 3-4 vertical buffers open. Being able to fit all my code in those buffers is a godsend.