Roger does a nice job of summing up my opinions on the issue; having to scroll up and down is a great annoyance. I run into the problem frequently when dealing with PDFs.
He lists a number of circumstances when using multiple columns might be tolerable. The problem is - how can the author know when those circumstances are occuring?
So great! No scrolling needed. Now imagine you use a screen magifier…
…and there we have scrolling (up and down and up and down and up and down and in cyberspace nobody can hear you scream).