Hey to each his own concerning boat covers. I make boat covers and there are many grades of boat cover. Some are for stationary use, and some can deal with 65 knot winds going down the freeway!
There is more to protection than just dirt. UV damage is the number one source of damage to sails, running rigging and gelcoat (and boatcovers). A well made cover will keep the boat clean, stop sun damage and has as much ventilation as the owner desires, vents should be sewn into the cover at a couple of intervals. Most premium acrylics are semi breathable as well. If your boat is dry in the first place and stays dry under a well made cover, you don't need fans to remove water vapor.
Another technique for keeping a cover on a boat is the one I borrowed from wakeboard powerboats: the cover is held on with ratcheting straps that completly encircle the perimeter of the boat. The sides of the cover are brought down far enough to take advantage of the curvature of the hull, and a loop is sewn around the lower edge for the ratcheting strap(s).
Unfortunately for the bargain shopper, my premium boat covers, with a 5 year waranty, cost more like $1200, include vents and access zippers, and includes any design features desired by the owner, within reason. I am measuring a 26M tomorrow for a cover, the boat has roller furling and is brand new. I plan to encapsulate the boat and furler/mast.