> But there are many cases where code works best as a spreadsheet.
Indeed, that's true, and I don't mean to be too much of a proportional font fanatic. Even if I try to avoid coding styles that require monospaced fonts to be readable, there are always cases where it helps to get a monospaced view of the code.
One feature I really like in an editor is an easy way to switch between proportional and monospaced views. The best support I've seen for this is in Komodo [1]. Like many editors, Komodo supports multiple themes of font face/size/colorization/etc. What it does differently, though, is that each theme has two font selections, a proportional font and a monospaced one. So you can pick your favorite for each, and then choose whichever works best for the particular code you're looking at right now.
Oddly enough, out of the box Komodo doesn't give you a keyboard shortcut to switch between proportional and monospaced, but it's easy enough to add a keyboard shortcut for this. (In the Key Bindings dialog, look for User Interface: Use Fixed/Proportional Font.) I have this set to Alt+O which is unused in a default Komodo installation on Windows, and it's easy to remember because "proportional/monospaced font" has a heck of a lot of "O"s in it. :-)
But when I do use an editor that only offers monospaced fonts, it's not a terrible thing. I use your excellent Zeus IDE [2] for my Go coding and it's a pleasure to work with. Sure, I miss my proportional fonts, but having the integrated debugger and other goodies for Go makes it well worth it.
[1] http://www.activestate.com/komodo-edit
[2] http://www.zeusedit.com/