I don't see the math (weight) nor functionality of a bivy.
First of all, a bivy bottom (used in lieu of a groundcloth), made out of some kind of WPB like silnylon, is almost by definition going to be heavier than window shrink wrap used with a floorless tarp.
By definition, I mean it has to be strong/heavy enough to sew, even @ 1.3oz yrd2. Compare this to really lightweight WP window shrink wrap, which has good puncture resistance, but almost even better, is so cheap ($2-3 for a 7x4) that it can be discarded after a trip.
Secondly, the incremental increase in tarp weight for a full coverage tarp (eg tapered/cat cut 10x8) vs bivy tarp (7x5) is nominal, especially considering you still have to take the same poles, guys, stakes, etc.
Third, coverage is a function of square footage. A 5x7 tarp will cover 35 sf, whereas a tapered/cat cut tarp, starting off @ 10x8 will come in the range of 65-70 sf - nearly 100% greater coverage/protection.
Fourth, if you're using a bag/quilt (either syn/down) built out of a quality DWR, like Teflon coated M50, in combination with a full coverage tarp, then you already have good splash resistance. That is, if any rain was to find its way in.
Fifth, using a quilt/down without an additional layers on top and/or a closed WP bottom (like sil) cuts down on condensation.
Sixth, using high quality insulation (eg 900FP down) will achieve much greater warmth with the addition of 2-4oz vs 4-6 oz of bivy material. If you want extra warmth or want to avoid drafts, simply make the quilt bigger and/or overstuffed. Either option will weigh less than a bivy.
The only advantage I see to using a bivy is the weight saved by using a poncho tarp vs carrying a stand-alone 5x7 bivy tarp. Yet, this 'advantage' also has a drawback - who really wants to hike in a poncho if it's raining? And do you bring along a wind shirt for marginal conditions?
I think many who have experimented with bivy tarp and/or poncho tarp combos have come full circle back to the tent concept, in terms of full coverage, but are simply using over-sized tarps to fill the 'tent' function. That's me.