This "to cover or not to cover" issue has had my rusty mental gears working overtime for the past couple of weeks. The "experts" seem to agree that using a full body cover can cause more scratches than the benefits are worth. But a couple of people I know who actually use the things swear by 'em. The downsides of not using a cover if the car has to be parked outside are plentiful. In my area there's: pollen (sticky, yech), "kousa" ("yellow sand" that blows over from China occasionally, also yech), dust kicked up by agricultural activities, bird poop ... etc., etc. And snow! Since snow is basically a bunch of tiny little ice crystals, I've found that brushing snow directly off a car is enough to cause scratches.
So, to make a long story a tad shorter, I bought a cover. I figure that as long as the car is kept clean and the cover is used properly, using it is probably a better option that not.
Will report back if I manage to prove myself wrong. 8)