Proportional fonts have less mismatch when used for code, but they still tend to look "wrong" to me. A font like "Input" is attractive and offers proportional and monospaced versions. You're still left with the larger problem: fighting the font to line up data across multiple lines.
Maybe you don't care about that, but in both my hobby and professional projects, I've found that alignment makes the code easier to read and easier to work with. I would need a very compelling reason to abandon that, and I haven't seen one offered.
Going back to your original comment, you said:
> Proportional fonts are so much more legible and nicer to look at.
You also provided a convenient reply, which I think applies here:
> You should just use better fonts.
Hack, Input Mono, and a zoo of others are very attractive monospaced fonts. You aren't stuck with whichever ugly ones you're thinking of.
Yes, you give up the ability to do ascii art in your code. I find that this doesn't really matter to me or much of the code I use professionally (I can open up some random javascript code off of GitHub and they don't do non indent alignments). I guess if I was programming in C, this might be much more of a problem.
Aside from that, in the stylistic sense, it's like reading a newspaper article in cursive or an academic paper in comic sans.
Those are the things that struck me when I've pasted code into an editor with proportional fonts, anyhow.