Usually it involves alignment issues. Some people neatly align array entries, etc, and variable width fonts makes this look ugly. Also, people often put ASCII diagrams, etc in the comments, which are screwed up by non-monospace fonts.
> This could even (re)open the path to the Holy Grail of literate programming!
The Leo Editor[1] is your friend. It's the only such editor that seems to do it well. And by well, I mean "poor experience, but better than all the others."
