When I think of the number of times I've had to set up or take down my shelter while it's raining hard, and of how many times I've had to leave the tent during the night when it's raining, and of how many times I've had to hike in horizontal rain.... No thanks, I'll endure the extra weight of separate shelter and rain gear, and use a jacket, not a poncho. It also would be pretty hard to fit both my dog and me under such a small tarp.

Your mileage, of course, may vary!