If I have a long, complicated comment, I don't like appending a series of patches to it. It makes it less clear instead of more clear. This goes doubly for posts.

If a comment has been replied to, it's good to highlight changes with eg <EDIT>... </EDIT>, if they change the interpretation of a reply.

Both sides have merit. If LW continues to grow, it will probably reach a point where I'd prefer freezing edits after 24h, because it will have a higher proportion of users who abuse the ability to edit.