This is a debatable suggestion.
It has Pros and Cons, so let's discuss.
Pros:
We could easily fix spelling mistakes without adding more posts and bumping threads.
We could help the appeals section, people always forget the template so people could edit them and add the template properly again without adding to the thread and looking like a moron.
People could edit a post and add updates to it, easily.
Cons:
Remember that guy that reported a guy for a mute, and in the picture he had a hacked client? if we had the editable feature in, we couldn't have seen that.
People who say things, like racist terms and swearing. They could edit it and change it, like it never happened. no proof at all.