I often use ASCII art in comments to add explanatory detail. That would become nearly impossible to do correctly with a proportional font.
Another thing is easily formatting tabular-looking data (e.g. array initialisers) reasonably well. Proportional fonts mean this can't be done, and existing formatted data looks like a mess. Moving the cursor vertically with a proportional font also causes it to distractingly jump left and right instead of in a straight line.
Has anyone who prefers proportional fonts considered whether they'd like proportional vertical spacing (i.e. the height of each line varies depending on what characters are in it)? That would be the other extreme of variable spacing.
If one were completely sold on the idea of proportional fonts for coding, it would make sense to go to fixed tabstops (like a word processor or typewriter) rather than fixed-width tab characters, so tabs could always be used to align tabular data.
Another thing is easily formatting tabular-looking data (e.g. array initialisers) reasonably well. Proportional fonts mean this can't be done, and existing formatted data looks like a mess. Moving the cursor vertically with a proportional font also causes it to distractingly jump left and right instead of in a straight line.
Has anyone who prefers proportional fonts considered whether they'd like proportional vertical spacing (i.e. the height of each line varies depending on what characters are in it)? That would be the other extreme of variable spacing.