This is probably a snarky reply, but here is the serious answer: proportional fonts, with appropriate kerning, is a lot more legible than monospaced font. There is a reason why the press moved into that direction once it was technically feasible. But the same people that bring books as an example why 80 character line length should be enforced would gag at the notion of using proportional fonts for development. It just goes to show that none of these things actually matter, it’s just legacy patterns that remain in-place from sheer inertia, with really very little relevancy today other than the inertia of the past.
I agree. I've tried coding in C-like languages with proportional fonts a few times, and punctuation ends up feeling cramped, hurting legibility. We need more proportional fonts for programming where punctuation gets the same size and spacing as in monospaced fonts.