I think it's important to have a post edit timelimit to ensure/maintain the quality of the threads.
After a few days, a post could have pages worth of replies, where people have spent a lot of time and energy thinking about what to write and posting. If there is no time limit, then the original poster can come back, delete his post, and leave a huge thread that makes no sense with the key questions taken out.
For example, suppose Pook decided to delete his posts because he's sick of this place or for whatever reason, how would the DJ bible look then? And what about all the people who spent so much time replying to him?
Bottom line, the posts here became the property of this site, therefore, they should not be so easy to delete. So don't post anything that you are not 100% sure you'll want to remain here. It's only fair really. It's like sending an email, once you've sent it, you cant go into the recipients inbox and delete it. If you're not sure, then don't send it, or don't post.