I just did a 5 day backpacking trip on the PCT in Calif. My buddy used a Bivi. The first night he liked it but the other nights he didn't. He said it could feel the wind a lot more with it and if it would have rained he would have gotten soaked.
It was lighter and he was able to pack it up a lot faster.
Personally I like the tent, I know it's dumb but in a bivi I would feel like a bear burrito.