While I wouldn't say it's THAT significant, there is one other advantage to fixed-width fonts that I've experienced. Due to the fact that they align vertically, sometimes they expose mistakes if you're constructing data in a table-like fashion. Or when you have a bunch of copy/pasted lines that differ only in the parameter values. Like, "Hey wait a minute... this line is 1 char wider than the previous one. Oh, whoops, I fat-fingered something there".

I wish I had more specific examples and could quantify it better, but I know I've caught quite a few errors this way.