[wanderr]

they do disappear in one sense: the link to them sent out in the email telling me about them goes away. And if I have a bunch of collapsed comments, it's annoying as hell to find the right one. This is usually an issue when the developer says something like:

"ok, I fixed that, but what do you think about xyz?"

Now I have to go to the comments and expand 35 of them until I find the right one so I can respond to their question.

It's also sometimes the case that changing a line is not the same as addressing a comment, or maybe it only addresses part of a comment, or maybe they just changed whitespace because now all that code you commented on is inside an if...