The machine will be fine, but the cover may get weathered looking. So it depends on how long you want the cover to look good. Storing it under the "roof" of the dock helps it last a lot longer.
Otherwise you would have more days of sun fading, and sun weakening of the fabric. More chance of wetness and mold. Those who want to store their machine with just the factory cover and no other weather protection could go buy an additional cheap generic cover to use instead to keep the factory cover looking good. Or wrap it yourself with a tarp. Or just budget for a new factory cover every N years.