A mold breeder unless you constantly run a fan to ventilate it.
Get your car as high up as feasible on 4 sturdy jackstands. Cover it with a light, breathable cloth. Every few weeks, or whenever above freezing, uncover it, start it up, and run at a fast idle 'till hot. If you have good stands, you can slow it down and run through the gears. The next day, after it is dead cold, re-cover.
For the price of that thing ($390 ) you can buy a floor jack, the jack stands, a nice cover, and have money left over to take the wife to one of her favorite places, like a tractor pull, boat show, or swap meet.
You might want to check out http://www.carcapsule.com. It has a fan to keep air circulating to prevent/minimize mold/mildew. It's for indoor storage, probably wouldn't do well outside.
I know of a bunch of these in use.All except for one are being used for motorcycles.The bikes look spotless in them after being left in a cellar for the winter.I know of one with a fan on it,that blows constantly,uses almost no juice though.He said while it works nice,he wouldn't bother with the fan one again.He says there is just no need for it,the blow up and seal ones he has have been working just fine.
I have a buddy who kept his race car in one of these (not sure if was same brand, but still a plastic zippered bag) Stored it for the summer months, when he removed it in the fall the pearl white paint had millions of small cracks (like spiderwebs) and had turn a dengy yellow on hood, roof and trunk.
I would cover the car with moving blankets rather than let the plastic touch it. I'm going to cover my runabout with moving blankets then with a plastic tarp. Plastic on paint is bad, and it's probably bad on any polished metal also.