What's the point?

What is the point of perma-links in blogs if they aren't truly permeant? Many times I've looked at some blogs like Scripting News and seen an interesting point. Nothing major, just a comment like the one the other day about plans for taking the afternoon off and taking a drive. Later the content is removed. I'd like to figure out what the rules are.... When is it proper to de-link text? When are things considered "here to stay".

On this site, once things are posted they stay. They might not run on the front page any longer or they might have to be modified if for example someone claimed infringement (unlikely). But there would still be text letting the reader (or re-reader) know they aren't loosing it.