There is a long tradition of light weight backpackers using poncho/tarps. Ponchos can provide triple duty: rain protection, pack cover, and shelter for a very modest weight... but there are downsides as well. Personally, I think most poncho / tarps are too small for adequate shelter.

Poncho as shelter: The down side of the classic poncho / tarp is that in a serious storm there is little protection from blowing rain. It's common for poncho/tarp users to bring an ultra light bivy to protect against blowing rain which obviously raised the weight of the system. Other other issue is what to do when you need to leave your shelter for chores or "the call of nature". Options are go naked and dry off when you get back, use a DWR windshirt which can provide adequate protection for a short time, or bring a second rain item like those $1 plastic emergency poncho. I used a poncho tarp for around a year, but eventually switched back to slightly larger shaped tarp (and rain jacket) which only added a few ounce to my pack while giving me significant better protection.

Poncho as rain gear: in theory have good ventilation, can are be very light weight. Many people like that a poncho can be put on without removing a backpack. and can do triple duty: rain gear, pack cover and shelter. While the multiple use nature of a poncho is attractive, I have never been happy using a poncho. Disadvantages of using a poncho are that many people find them ineffective in high winds, awkward when bushwhacking and impossible to use when climbing. My personal experience is that my chest get soaked in sweat and condensation and my sides get damp from wind blown rain. I have tried ponchos for three different times, each six (or more) month in length. After each poncho experiment I switched back to a rain jacket.

The Six Moon Designs Gatewood Cape does look to be one of the most protective shelters made from raingear. It seems beloved by it's owners.