> Separately from the "lining things up" argument, though, there's an argument to be made for characters in any editing situation to be wider, even if they're not all the same width.

Have a look at Input [1]: a TrueType font built for coding, with somewhat-wide I and l and super-wide punctuation marks.

The issue is one of picking the right font, not of monospace vs. not - street signs, for example, have long used different fonts from printed pages, since they demand high accuracy and fast recognition of short chunks of text at long distances, as opposed to the pleasant long-reading experience demanded from most body fonts. Most systems just happen to have built-in variable-width fonts that are terrible for coding.

[1] [http://input.fontbureau.com]