Good replies that I would totally agree with, especially the damage that can be done from icicles and ice slides off the roof. One issue not mentioned is that when a full cover is used it not only holds moisture it makes the perfect habitat area for small rodents. Each year doing clean and checks I found high voltage contactor areas stuffed with nesting material and the occasional fried mouse. A few resulted in smoldering fires that melted wire insulation. Not that big of an issue if you clean it youself or pay for someone to come out and check the system prior to spring start-up.