Reading monospace text sucks.

It's only good for coding for alignment issues. But reading monospace characters for long blocks of text is hard on the eyes. (Not to mention that proportional spacing is more efficient -- you can get more text on a line with roughly the same letter forms.)

I've taken to setting whatever IDE or editor I'm using to have comments in italic Helvetica, rather Menlo, my fixed width font of choice. It makes comments easier to read, and I find myself writing slightly more of them as a result.

It's not about reading, it's about writing. Many measures in writer came from studying coding programs.

I dunno about you, but I barely type when I'm coding. I'd guess about 90% of my day is reading and thinking.

We only use monospace because there is a lot of symmetry in code, and unlike other forms of writing, a single punctuation characters can completely change the meaning of the text (/.!^,;).

Most of the need for monospace in coding is for the sake of indentation and alignment. If you have any ASCII diagrams (see the Internet RFC's) you need monospace.

But we forget how much of this is a circumstance of history. Teletypes were monospace, like typewriters, for mechanical simplicity. Line printers and DecWriters were monospace for the same reason. Then the first video terminals and eventually the original IBM PC were all monospace, all for the same reason, and because of familiarity. We simply got used to monospace and came to depend on it. There is no inherent virtue of monospace.

Come to think of it, the first (mainstream) computer I remember with proportional fonts was the original Mac in 1984.

The idea of using a monospaced font was to invite the writer to conscious slowness. Reading typography needs speed and ease, writing needs care and precision.