Let me preface I've never thru-hiked anything nor have I ever used a bivy. the Poster above seems more knowledgeable on thru-hike needs. However, if you feel you absolutely want to carry a tent, check this out:

http://www.lightheartgear.com/index.php?...2b7a2b043aa8440

This tent is on my wish list. May be another year before I pick it up, but I like the weight, look, and size of this tent.
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It is one of the blessings of wilderness life that it shows us how few things we need in order to be perfectly happy.-- Horace Kephart