When I have to do a big descent in the rain, I zip up. That's because I'm wearing enough clothing so that while climbing in a pouring rain I have everything unzipped down to my undershirt which currently doesn't have a zipper. If it did, I'd unzip that, too. In fact, that's how I know I'm dressed right. That said, descending from Rainy Pass in the North Cascades in the sleet can be intimidating. The good thing is that it's usually a lot warmer lower down during reasonable touring season. No hotel up there.