The benefits of covering an air conditioner include:

• Keeping the air conditioner coils cleaner, so your unit will run more efficiently when it’s turned on again next season.
• Covers prevent yard waste,such as sticks, leaves and other things from blowing into your unit and causing damage.
• Covers can prevent water from resting directly on coils and freezing, which could be damaging, especially if there are a lot of freeze-thaw cycles over the winter. Something pretty common during a usual Rochester winter. 

The potential reasons to not cover a unit include:

• Covering a unit with a plastic cover will trap moisture inside, creating a high humidity environment, by creating a bit of a “greenhouse” effect for your unit when the sun beats down on the cover. The electrical components, and circuit boards hate this sort of environment. To address that lack of air, look for a cover that “breathes” like canvas or has a liner on the inside to absorb some moisture and minimize the collection and dripping of water.
• Most manufacturers don’t specifically recommend covering the outside unit.