Scenerio...
A member posts a lot of very useful information that is of benefit to many people. The thread grows as others contribute valuable information. Over time, it becomes a well liked resource on the forum and many people have contributed into it.
Later, someone pisses off the original starter of that thread and they decide to give the community the finger, pack up their toys and go home. When they do, they delete that thread because they started it and the software saw that they had appropriate permissions to do so.
Everyone loses as a result.
Still think it's a good idea? I don't. We have moderators for a reason, and that's so that not everyone has keys to the kingdom that they can manipulate at will. This is the way it will stay.