Well... there is a little more to a good cover than that I am afraid Paul. A good cover should breathe - and not promote condensation on your grill. Nothing makes a sweaty greenhouse like a plastic bag! 
I can't agree with this, Tee. It's true if the item being covered is generating water vapor, like a person or something that contains evaporating liquid. Then the vapors are trapped by the cover and they condense under it.
Think about this—nothing breathes better than no cover at all. Every morning, well after sunrise in humid Nebraska, my grass is as wet from condensation (dew) as if it were watered. If breathing were important in reducing condensation the grass would be dry 'cause it breathes pretty darn well without a cover.
One morning as I was testing for grass dryness in order to start mowing I checked the two grills on my deck that are covered with plastic bags. Both bags had heavy dew on the outside—actual water droplets that would run at the touch. Both were dry underneath. I had the same results when checking on several consecutive mornings.
So I'm not convinced that all covers need to be vented or breathable.
