Usually an observant user will alert a moderator with a complaint and then that moderator will take care of the post. Generally, if it doesn't break any set in stone rules and nobody is offended, then the job of moderating is easy. At least in the forums I moderate, the users who post there are very good at complaining and catching things quickly. Well, someone has to push the button to make it happen so that's what I do to keep the trash out and the information in. Just because a post isn't caught in a certain amount of time does not give that post a free-ride from deletion. If it doesn't belong or it upsets the flow, then eventually it should get deleted.
Eventually, yes. But eventually can take days. In the meantime, the post gets quoted and so on and there's little point in deleting it.
One example of where this would be handy would be in creating a series of threads like the GEMS series. I was only able to create / enable those threads because I happened to be a moderator at the time.