I am mostly a multi-day solo hiker and it has almost got to that stage of bivi or nothing for me. I guess conventional wisdom would say to bivi over one night and use a tent on multi-day, but I am reverse.
The disconnect that a tent brings is not something I always enjoy. Although it can have its benefits. Instead I carry a bivi and tarp. It has nothing to do with weight, so if anyone is thinking of bivi just for weight loss, don't waste your time, tents are light enough these days.
I prefer to bivi because I can see the sky, I can be outside. That's the experience for me. The tarp is a precaution that I only use if it is raining or excessively windy, otherwise it stays in the pack. The bivi also gives me more freedom to sleep more diverse places. I slept on sand dunes next to a beach on a slight slope. This summer I slept on a rock summit of tres picos in Argentina, I have slept in urban areas (airports for example), on crib goch, and numerous other places where a tent wouldn't work.
On occasions of one nighters, I will sometimes use a tent. Admittedly it is nice to hide away in your own little space in the middle of nowhere.
I would say two thing about using a bag:
Firstly, it is obviously not for everyone, it can be frustrating and uncomfortable, and annoying if you are someone who carries a lot of kit.
Second, I think hooped bivi bags are almost pointless. They are more like ultra-compact tents. I don't think they have the room and comfort of a full tent, but lack the true versatility and ease of a regular bivi.