My Silver B has been outside for three and a half years with the Weber cover. The warming grate has rusted a bit (but maybe that's because I neither use it nor clean it). Beyond that' it's in great shape. The Weber cover is vented, which I believe is intended to allow condensation to escape. It doesn't seem to let in lots of water, even at the vented area (this may be because the vented area is kept away from the body of the grill by the handle).
Even leaving it outside, it will still outlast you (unless you have it on the beach).
Even leaving it outside, it will still outlast you (unless you have it on the beach).