I'll just point out that, compared to your 10 to 15 oz inner nets, my SuperLight bivy with cuben bottom weighs less than 6 oz and doesn't require any fussing to install. And you can use it with any tarp that you care to, rather than just the one it's fitted to like the inner net. And it arguably helps you sleep more warmly. Until you start talking about really huge mids it's always going to be lighter- though clearly more expensive- to give everyone an individual bivy than to have the inner net.
But OTOH, before I talk someone into something for which they aren't prepared, the big downside of a bivy is condensation. If you aren't careful you can perspire yourself into total down collapse. My bivy has the optional larger netting window for better ventilation. In really cold weather (i.e. when the dewpoint tends to be actually INSIDE your down bag) you can mitigate this with a VBL liner or clothing. Of course a VBL sleeping bag liner is 5 more ounces, thus negating the weight advantage (but still a hell of a lot warmer).
On the gripping hand, if you're using a mid you probably really don't need protection from rain spray- you just want a ground sheet and bug protection. For that, there are bug bivies like the MLD version, 6 oz (5 oz in cuben). Not much of a condensation problem with a mesh bivy...
Of course bivies work best for people like me who do nothing in their shelters but sleep. If you camp more than hike you probably want the inner net.