Necessary no, preferred yes. There is no doubt, from my own personal experience (I have done it both ways) that keeping one covered will keep it cleaner, seals and rubber in better shape, and decals from fading. Keeping one uncovered just requires more preventive maintenance and cleaning. So pick your poison, spend money on a cover and spend time taking it on and off after each use, or spend your money and time doing a little more maintenance. Just my opinion. Also, the longest I have ever gotten out of a cover is 5 years. Usually they last me about 4 years. Funny, because that is when the warranty ends on them. Go figure...