You've probably seen some code in a variable-width font at some point in your life, and just never were conscious of it because it looked so right.

It's alot like syntax highlighting: people will lose their minds if you suggest that it's even possible to live without it, but the fact that they are able to read their shell input line, their less/vim :-line, their google input line when they use quotes/minus/etc, their books with code in them, ..., suggests that their complaint is not based on a _need_ for highlighting but on an expectation for it. It's the only way to explain why it's apparently necessary in one program and completely unneeded in all others.

Fixed-width fonts are like that.