I'd like to share my experience with car covers.
My 1974 Spider is in very nice condition with very good paint (about 15 years old).
The car lives outside and has been my daily commuting car for the past 3 years. Modern car covers have been my (and the Spider's friends)
When choosing a car cover for outside use, make sure it is breathable, has built in air vents...like a tent, and that it has straps that secure the cover when the wind increases.
In my experience a car cover is not only one of your best defenses against corrosion by preventing water collecting in channels, doors and sills, but it keeps the Spider clean, so that you seldom have to wash it. A good cover in a light colour will also reduce UV damage and keep the car cooler than if it were left exposed. The car covers do not appear to have worn the paint thin or been abrasive on the mostly curved panels of the spider.
If you are using a car cover, do try to keep the car clean, so that dirt is not trapped inside the cover. I cover the car every night and during the day while it is parked at work. Covering the car only takes a few minutes and it soon becomes routine. I suggest that you carry a large towel in the boot so that if the cover is wet it can be placed on the towel.
A good car cover is well worth the expense and if it makes the difference between driving your Spider to work or leaving it in the garage it is a very small price to pay (and your car will thank-you).