The problem is that beyond a certain width it becomes infeasible for the eye to correctly jump back to the start of a line. There's some research on this, but I can't find it at the moment.
Smaller screens obviate the need for this, some of the time. But I've seen a lot of layouts which just compensate using white space - that fits with current design trends, but tends to mean I have a 27-inch monitor with a strip of content down the middle. It looks clean and minimal, but I suspect columnar text will come back into vogue at some point, and offer greater information density.
http://www.usability.gov/get-involved/blog/2006/08/line-leng...
However, columns are 2 dimensional, meaning that skimming back and forth between them requires more complex spatial processing than scrolling up-down.
I need to find the link, but I saw a study concluding that it is mostly useful if you are a fast-reader. Slow-reader tend to be more at ease with one column. So it might depend on the persons reading.
[1] http://graphicdesign.stackexchange.com/questions/3553/why-do...
It is one of the requirements of super speed reading: http://en.wikipedia.org/wiki/Speed_reading
Basically you could train your peripheral vision to read too, not just your fovea. You also train your brain to put them in a buffer and make sense of it. Imagine a cone around the point you focus your vision. Instead of just one word you could read 20 or 30 at the same time. Your brain makes sense of it very fast(with short lines).
Your brain could read multiple lines at the same time and make sense of it, because it is ordered. With long lines the words under another word have no relationship at all, so the buffer has to be enormous, in practice you can't read multiple lines at the same time if they are long.
I could read super fast only(eight times faster than everybody else) if the content is formatted.
For me is so frustrating I created my own tools just for reading.
This has been know for a long time, in the typographers and designers world, formats like pdf have used this since their inception.
But programmers are a different beast. Web pages were functional but ugly and only now they are getting the functionality of pdf-latex twenty years ago.