Do I really need a winter cover??

I have had small AG pools and never bothered with a cover, I live in the far north and I have always floated some bottles in the water and shocked it. I have installed a larger pool (18ft round) I know thats not huge but would the same methods of winterizing work ? I am bringing in the filter and pump so my only concern is whether I really need to cover it? The pool will be completely frozen for at least 4 months in the winter. We usually get a good 2-3ft of snow over the winter so by spring time there is no need to put water in the pool, so I just have to keep an eye on the level in the spring and make she it doesnt start overflowing into the skimmer. Anyway Thanks for any advise you might have!
Thanks
Glenn

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.