Something else to consider is that older bicycles are less able to deal with inclement weather than modern ones, which have sealed bearings, plastic lined cable housings, etc. So, if you choose to use your older bike in wet conditions, it's going to inevitably suffer wear and damage unless you are continously maintaining it. You need to decide how special you consider your bike to be, and accept the fact that you can't have your cake and eat it too.

As to your original question, I agree with others here that the cover you are considering is about the worst thing you could do. It would just trap moisture and optimize corrosive conditions.