Instead of a tarp consider lightweight windshirts with some water repellant. Patagonia Houdini is very popular with backpackinglight members and highly rated by anyone I've ever known. My XXL weighs in @ under 4oz. They are not water proof but will not wet out in anything less than a complete downpour and even then it takes some time (over 30 mins), so just be in the trees or moving back to camp. It doubles to help keep you warm on a cool day, or windy day. I can stay warm in my Houdini w/a 5oz Montbell down vest over a lightweight wool shirt down to just above freezing while backpacking (moving).
A tarp is not very multi-purpose if you already have a shelter, IMO. You can optionally add some windpants (GoLite has some on sale now) for an additional 6oz and have a pretty much waterproof suit for 10oz that can also be worn as needed depending on weather conditions.
