http://www.stroustrup.com/3rd_notes.pdf

> At first glance, this presentation style will seem ‘‘unnatural’’ to programmers accustomed to seeing code in constant-width fonts. However, proportional-width fonts are generally regarded as better than constant-width fonts for presentation of text. Using a proportional-width font also allows me to present code with fewer illogical line breaks. Furthermore, my experiments show that most people find the new style more readable after a short while

Great book, and I have used proportional fonts occasionally. It's much much better than you might imagine (as long as you are sane and use tabs for indentation). Obviously you can't get "perfect" alignment like you can with fixed-width fonts, but it is certainly nicer to read, so there is a trade-off.