I was asked by PM why I don't like bivy bags, and half way through writing the answer I thought I should post it here rather than by PM. You will note that a lot of them have to do with how my wife and I travel, but they are not the sole reasons.
* Two cramped confining bivy sacks weigh as much as one comfortable tent.
* When it is cold we snuggle up together to share warmth: works in a tent.
* Changing out of wet clothing in bad weather is dead easy in a tent; it is horrible in a bivy sack.
* In cold weather bivy sacks tend to get condensation on the inside, which of course transfers back to the quilt(/SB) shell and insulation. In a tent the condensation is MUCH less, and is on the roof, far from the shell.
* In bad weather you need to be sheltered for >12 hours. Try lying still in a bivy bag for that long! dead easy in a tent.
* Try cooking in bad weather in a bivy bag. You may set it alight. It is dead easy in a tent, even in a howling snow storm.
* It is 3 am, the weather is awful, and you need to go to the loo. Not hard to get dressed in enough gear in a tent for a brief trip outside, but try that in the dark in a bivy sack.
Enough? I am sure I can think of more.