One could shoot in the rain without a rain cover thousands of times and never have a problem. On the other hand, the camera could malfunction the first time a drop of water hits it. It is the luck of the draw. As long as a drop of water doesn't get to the wrong place at the wrong time, you won't have a problem. If the drop of water get to the electronics, the damage could be extensive. Each of us must decide how much risk we are willing to take. I consider a quality rain cover, cheap insurance.
As for the plastic covers, you get what you pay for.