A stand-alone bivy is a very specialized piece of equipment and is great when needed. The advantages are 1) can sleep in just about any small spot, after all they were designed for sleeping on a ledge half-way up a cliff, 2) compact so this is my shelter of choice when I want to stuff a week trip into my 2400 capacity pack, 3) high winds above timber - many who climb Mt. Rainier and camp out have had better luck in a bivy than having high winds flapping their tents all night and 4) viewing stars at night. Be warned, that MOST people do not like a bivy. If you use a bivy, then only the basic ones that weigh about 1.5 pounds are worth it. If you get a fancy one with poles, you may as well get a light single tent. A bivy will also cost you about as much as a tent. I would get a tent first, and then maybe add abivy after you get experienced.