As much as I love bivvying, I agree with TJ to some degree. My lightweight tent is only a few grams heavier than the bag, can sit up in it, cook in it, get changed in it, etc. If I was out for more than one night I’d take the tent.
For me the bivvy shines as it does feel more ‘earthy’. It is quick to set up and leave, it ain’t going to blow down, and it doesn’t need a tent size area to squeeze into!