While I wanted to use just a tarp outside of bug season, it didn't work out because my dog was restless all night long. He needed the tent for security--he considered it his crate. Since I wanted my sleep, I stuck with the tent. Now that he's gone, I could, of course, switch, but I'm looking for another dog.

From my limited tarp experience and talking to regular tarp users, it would appear that with a smaller tarp there will be "splash" with heavy rain and you'll need a water resistant (NOT waterproof) bivy to protect your sleeping bag. With a larger tarp, you can snug it down closer to the ground and can do without the bivy. Everyone's preference may vary here, but the total weight will be about the same. The tarp I tried for me and the 75-lb. dog was 8' x 10' and there was no need for a bivy.

I also found that by the time I added together the weight of the tarp, guylines, bug net and ground sheet, the total weight was the same as many lightweight tents, including the one I was already using.