Writing down comments on edge cases doesn't make the code more concrete -- comments don't change the code.

Ah, what I was trying to say was that the writing of the comment helps make the thinking more concrete: often I can think I have something clear in my mind, but when I have to write it down, I find that my thinking was sloppier than I thought... if you see what I mean.

if the comments get written during development, that's fine.

Absolutely. This is what I was trying to say when talking about the comments being an integral part of the programming process. You have to think a chunk of code (or data) is for before you write it... writing some of that down in the form of comment helps the thought process now, as well as having benefits later.

The writing of comments also has its place when reviewing the code. First, again because the act of writing concentrates the mind. Second, if the code and the comments don't tally, something needs to be thought about !

I must say I'm taken aback by the response... I get the feeling that there are a number of people who've been traumatised by being forced to eat their greens. Suggesting that greens should form part of a balanced diet seems to produce a gagging response :-)