Like oldranger, I have used ponchos as shelters. But I was guessing that I would not be caught in a hard rain. A bivy may be a good alternative. I have used those as well. There are emergency bivies available that weigh less than 8 oz and are multiple use. There are plenty of regular bivies available also.
When I went out counting on a poncho, I took two and used them together. Using a bivy is better.