This past summer I spend two months camping out and photographing in Iceland. We were faced with rain almost every other day, if not a slight rain EVERY day! Because I was there to photograph, the light rain was not going to stop me from making images.
Before going on the trip I waterproofed my darkcloth with two coats of ScotchGuard waterproofing spray, both the inside and out. With having the darkcloth, I was able to keep the WHOLE camera covered from the lens back, and still have it long enough to focus and compose the image. My bellows were often out to 24" or more, and they were protected completely by the large darkcloth drapped over. Clearly you could not do this with that little jacket. It would be impossible.
When I was using the darkjacket in cold weather, I often had the problem of my hot breath being trapped inside and fogging up my ground glass. If your ever photographing in cold weather, this is something you will want to think about. The problem still happens with a normal darkcloth, but not nearly as bad.