Theres a lot to be said for the new tents available now.
Many offer similar weights to the once advantageous bivy, where once they would have been twice the weight.
Even taking into consideration a groundsheet and tarp, though this advantage is mostly a thing of the past.
I believe the one advantage left of the bivy is just where you can utilise it.
Many is the stealth camp site I've had where I'd not have gotten away with a tent.
Also with the bivy I need a much smaller space to "put down" and can do it under much lower scrub compared to a tent.
Roadside (just into the trees/scrub) is very possible with a bivy many more places than where you could pitch some thing larger and you don't have to go as far to be away from prying eyes.
I've both and appreciate their varies strengths and weaknesses.
Theres a place for both.