Why using fewer columns is good

Less is more. Fewer columns feels simpler, bolder, and more honest. We’re communicating less information more clearly.

There’s also a by-product of the domination of centered layouts. Because we’re not filling the whole screen so much, and not trying to get as much on-screen at any one time, we simply don’t need as many columns of information.

source: http://www.webdesignfromscratch.com/web-design/web-2.0-design-style-guide.php