My dog gets dirty to fast, and I would like to stay as clean as possible. When I took my dog with me in my tent, I had to clean it out all the time and it was annoying. I was so jealous of my friend in their tarp. Even when it snowed that night, I had extra water weight on my tent and his little tarps was dry quickly after a 5 minute break laying it out.

I do agree thought that a bivy/tarp combo can be alot of weight, which is why I am doing lots of research to keep that down.