The problem with proportional fonts for editing code and related things
One of the eternal attractive ideas for programmers, sysadmins, and other people who normally spend a lot of time working with monospaced fonts in editors, terminal emulators, and so on is the idea of switching to proportional fonts. I've certainly considered it myself (there are various editors and so on that will do this) but I've consistently rejected trying to make the switch.
The big problem is text alignment, specifically what I'll call 'interior' text alignment. Having things line up vertically is quite important for readability and I'm not willing to do without it. At one level there's no problem; automatically lining up leading whitespace is a solved issue, and other things you can align by hand. At another level there's a big problem, because I need to interact with an outside world that uses monospace fonts; the stuff I carefully line up in my proportional fonts editor needs to look okay for them and the stuff that they carefully line up in monospace fonts needs to look okay for me. And automatically detecting and aligning things based on implied columns is a hard problem.
(I used to use a manual page reader that used proportional fonts by default. It made some effort to align things but not enough, and on some manual pages the results came out really terrible. This experience has convinced me that proportional fonts with bad alignment are significantly worse than monospaced fonts.)
This is probably not an insoluble problem. But it means that simply writing an editor that uses proportional fonts is the easy part; even properly indenting leading whitespace is the easy part. In turn this means that you need a very smart editor to make using proportional fonts really a nice experience, especially if you routinely interact with code from outside your own sphere. Really smart editors are rare and relatively prickly and opinionated; if you don't like their interface and behavior, well, you're stuck. You're also stuck if you're strongly attached to editors that don't have this kind of smarts.
(The same logic holds for things like terminal programs but even more so. A really smart terminal program that used proportional fonts would have to detect column alignment in output basically on the fly and adjust things.)
So I like the idea of using proportional fonts for this sort of stuff in theory, but I'm pretty sure that in practice I'm never going to find an environment that fully supports it that works for me.
(For those people who wonder why you'd want to consider this idea at all: proportional fonts are usually more readable and nicer than monospaced fonts. This entry is basically all plain text, so you can actually look at the DWikiText monospaced source for it against the web browser version. At least for me, the web browser's proportional font version looks much better.)