If you have leaves or debris that will fall into the pool over the winter, then why leave it open? If you do not have to worry about crap in the pool come springtime, then you don't need a cover. The cover just saves you from opening up to a gross swamp and fishing out a pile of junk in the spring - it also could help to keep the sunlight out, slowing down algae growth before you start up the pump in the spring.