Adding doors to MYOG Nightwing – Or just add a bivy?

The main problem with a tarp is sliding out from under it. Most backpacking country is verticalish. Doors don't help much with that. Some like a bivies for that reason, and they have good arguments in favor. I personally prefer to save the weight and pick camping spots more carefully. As for blowing rain, you can mitigate it by lowering the tarp's pitch