It changes the context of the discussion when you allow perpetual editing or additions. That's the main problem. You could start a big flame war, then go back a couple hours later to your inflammatory post and add (j/k) at the bottom, and make everyone who took you seriously look like fools for debating you, etc. If you have a thought later on in a discussion, it makes the most sense to add it to the thread at that time, signifying the point at which you had your thought and where it plays in the unfolding of the thread.
The way WHT handles this now is the best way, I think. 15 minutes to rearrange your thoughts properly, and then modifications via help desk only.