No, we've done it like that so that you can fix typos etc you spot after hitting the post button, but you can't go back and edit the content of your post once a conversation has started. If there's ever something you really wish you hadn't said, you can get hold of any one of the moderation team to ask for it to be edited or removed
No, it was misused several times in order to manipulate threads to change the meaning after someone else had answered. The moderator team first voted to remove the edit button totally, but due to popular request it was brought back for a limited time only