Earlier quoted context omitted.
It's more than that--I tend to lean towards words with the same number of characters for variable names which helps make things line up. Variable width would kill that too.
Do you think that you use fixed width fonts because you use words like that, or do you use words like that because you use fixed width fonts?
Using a variable width font would pose many problems to the editor and you'd have to have tabs on the "inside" of a line (as opposed to just having them on the front). That isn't necessarily bad, just much different from how I currently currently code.
And aligning to a tab stop is not always what I want--you'd need a tab character that didn't mean "go to the next tab stop" but instead meant "line me up with the corresponding tab in the previous and next lines at the minimum possible distance". That seems annoying and probably not worth it.