So many people don't understand that their browser window can be resized, so if you have text that spans the entire width, there might be 30 or 40 words per line, or even more. I've seen research that suggests reading is easiest with around 10 words per line.

Net result is that text is easiest to read with a small number of words per line, and the only way to ensure that cross-browsers, and for all window sizings, is to create a single column.

Here's a page using the full width: http://www.penzba.co.uk/Catastrophe/

That's quite hard to read compared with these:

+ http://www.penzba.co.uk/Writings/TheTeleportingTurtle.html

+ http://www.penzba.co.uk/Writings/TrappingATranscendental.htm...

+ http://www.penzba.co.uk/Writings/EnumeratingTheRationals.htm...

Personally I find those much, much easier to read.