I think it might be dependent on what language you use. In Perl, alignment is very important, as you have a lot of code that looks like:

   my $foo = Foo->new(
      named_initargs => 'go here',
      and_also       => 'here',
   );

(There are various technical reasons to avoid this style, but I think it looks too nice to avoid.)

As a result, a non-monospaced font would be a nightmare.

However, perhaps if you are using a language where everything is positional (Java, C), then maybe it doesn't matter:

    Foo foo = new Foo(args, go, here,
        and, if, you, run, out, of, room,
        perhaps, they, can, be, written,
        like, this);

Note that the only alignment is the initial indentation; one space is always as wide as another space, so a variable-width font won't mess anything up.

Obviously if you are using Haskell or Python, you will care even more about alignment, and hence will want a monospaced font. You will also want a monospaced font in your terminal emulator, as most console apps depend on all glyphs being equally wide.