[xigoi]

Code uses much more punctuation than prose, and punctuation is hard to discern in a proportional font.

[____________g]

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.

[jdougan]

Depends on which language you are writing in. Historically Smalltalk UIs use proportional fonts, and they work just fine.