I enthusiastically agree. I don't have time to keep a blog. If I did, I'd feel obligated to add to it and that's the last thing I need. The great thing about HN is that I just comment spontaneously when I'm moved to do so, and on several occasions this has led me to express thoughts that have been kicking around in my head for a long time, but which I never wrote down before. It would be nice to have access to the history of this.
It would be even more useful to have the full history of other users' comments too. One of my favorite things to do on this site is run across an intelligent remark and then look back to see what else that person has had to say.