I got three things for backpacking. One of them is a Mountain Hardware bivy. I slap my bag in there, lay down on the mat and I "survived" some pretty hard rain in that. It's really not that great when weather is hot but it is small and lightweight and with addition of nylon tarp shelter it is really all you need.
Mountain Hardwear Bivy

In Africa I used similar version of the above but it had a screen netting that made my bag bug proof and cover was more breathable.

Another thing that I use if I have more space is MSR Zoid tent. It is perfect when you know you really need protection from elements. Transition from big tent to small one person shelter is little harsh. You are wondering where all the room went and trying to change inside is really hard but packing wise it is so small that you won't even feel the weight.

I find myself hiking a lot with just a tarp and a bivy because it is more flexible. But that solution may not be suitable for everyone. I camped on an island that was so overun by bugs that I was glad I had my tent with me. And even if none of them was poisonous having things crawling in the sleeping bag is not the thing you want to experience in the middle of the night. Also on my outing this weekend temp droped to low 30s,wind chill made it even colder and it was also raining so my Zoid was perfect for this situation.

Matt