Cover questions are always entertaining on here... As you can see above there are two camps who are diametrically opposed
I am in the cover camp. I have had 4 different units over my life and every one of them has been covered. Most commonly I have used ADCO and each of them has lasted a minimum 4 years. My current cover is a different brand, which sadly I cannot remember the name of... it is a lighter material which made me nervous but three years in it is holding up as well as my ADCO's.
For me the cover is a common sense protector and a great insurance policy. The protection against sun damage on your paint and roof is money well spent. But for me the real life saver came when one of my sky lights cracked during the winter and was not caught by me until after the spring thaw. The water resistant cover meant that I had no intrusion from the broken sky light, without the cover I would have suffered significant damage.