Had an old one, finally threw it away, I just reverse the middle section, open one bottom vent and the top vent and put the water pan over the top, no rain , no mold...
I found that my weber 26 cover is very very tight and stiff as a board this time of year.
As smoke ninja said, it also holds in moisture, so I am using it over my UDS now, which fills with water if not covered appropriately.
They redesigned the WSM cover a few years back and I picked up one for my 22.5. It's a little bit more loose than the old one and it's a thinner material. I find it doesn't get as stiff in the cold weather and it breathes better letting any moisture that got in, out. You might give it a shot depending on if you have the older cover or not.