In reading a thread (say, over the course of 5-15 minutes), I will often want to post comments at various points. Because I am not in a popular HN timezone, if I needed to wait for comments to be endorsed, I would find this seriously frustrating. Waiting and checking to see if a comment had been approved would be a huge time sink.
Edit: I think that limit has been lifted to five now.
Edit: I think that limit has been lifted to five now.