A bivvy sack doesn't necessarily add weight to one's pack, provided equivalent weight is subtracted (in theory) from sleeping bag.

.I partly justify taking a one-pound bivvy by using it mostly with a one-pound sleeping bag with ultra-flimsy shell.. Taken together in benign conditions, this set-up is nearly warm as an equivalent two-pound sleeping bag, and much warmer in any sort of breeze.

The cover, with waterproof "floor," largely protects bag and its stuffing from mud, moisture, floods, snow, abrasion, etc. thus enabling a more carefree attitude toward sleeping arrangements. Separate slight plus: Stuff sack & stuffing become redundant..

Also, the bivvy (the top of mine isn't waterproof) can be used by itself, with or without some extra clothing, and/or with a winter sleeping bag, lending versatility to equipment.

Entering or exiting bivvy in the rain would potentially drench sleeping bag, and hanging out in a storm would be unpleasant and impractical. A one-pound SilShelter or any of numerous similar rigs would add lots of comfort in storms, along with a small bit of warmth.

Often if there's no rain I don't bother with a roof and with sleeping bag cover (bivvy) get nearly complete protection from breeze and muddy ground and insects. (Pad goes inside bivvy.)

In my view, nearly all bivvy sacks, including mine, are cut too small, and majority are too heavy and expensive.

A slim cut threatens to compress large volume of insulation (as in winter sleeping bag) and you can't easily store stuff in them once enscounced for sleeping. Adding a few inches would add weight, but how much?

Too bad more manufacturers didn't make a very light, jumbo-sized sack.

No experience with waterproof/breathable bivvies. Apparently the various fabrics vary with regard to breathability