The waterproofing of a caravan dependents such a lot on the mastic sealant joints. Even mastic manufacturers would tell you that it breaks down when exposed to sun and cold etc. Its a great comfort to me when I look at my van on snowy or rainy days, that its covered by a decent cover. Not only that though, they protect it from fading and brittle plastic due to the sun (have you noticed how older caravans are faded).
They can and do cause minor scratching to the plastic windows of the van, but I find these easily polish out. On balance, I wouldn't be without one - I include the cost of a cover when buying a hnew van, as long as its a good quality, breatable, fitted one.
wouldn't recomment a generally purpose loose fitting cover as i would imagine they can cause a lot of damage flapping about in the wind.