Unicode in code might be what pushes people to consider variable-width font. Just look at the simple arrow in a text editor (hopefully the text filter on this website thinks this symbol is OK): → (U+2192 RIGHTWARDS ARROW). It’s tiny. And I notice that the author uses plain `->`. You do need at least an “em” width for that kind of symbol.
But I wouldn’t want a normal variable-width font where colons are tiny and at-signs are gigantic. I read a programming email list in my regular webclient and it can be quite tiring. :)
It seems we're really close to making language specific fonts. Haskell would certainly be a prime candidate for this.
Is Hasklig not already such a font?