Adding a breathable bivy will not ~reliably~ add warmth. That’s not a benefit. You should never automatically plan on being able to use, say, a lighter bag just b/c you’re using a bivy.
However, one major benefit of the bivy approach is the ability to shed wind, and errant precipitation/sprindrift/external condensation, as well as manage internal moisture, in a layer which can be separated from the insulation for drying and storage.
Take those two general (if imprecise) distinctions:
1. internal moisture, which can be dampness in clothing, humidity in the microclimate of the bag, or the vapor which rises constantly out of your body. You want this moisture to pass through the system as easily as possible.
2. external moisture (precipitation, blowing snow, dripping condensation under a tarp or in a tent, or just humidity in the ambient air). Obviously you want to keep this out to begin with. At least, if it does soak into the outermost layer (which is managable, you don’t want it soak into the insulation).
Regarding internal vapor and condensation: An epic sleeping bag shell, under the right conditions, will trap water vapor inside the down, where it can condense. This can visibly flatten a down bag over 4-8 days easy, tho less or more time is possible depending.
With a bivy and a breathable, dwr-coated bag shell, the same thing can occur, but that vapor is more likely to condense just inside the bivy, but outside the bag’s shell. In this case, the shell of the bag can usually shed that level of external moisture reliably, until morning, when the bivy is removed, shaken off and briefly air dried if possible (a bivy can dry just as quickly as a good windshirt.) Then the bivy is stored AWAY from the insulation. With an epic shell, when you stuff the bag, you force moisture in or under the shell back into the insulation.
Either approach can work. I personally have used both, depending on conditions, but for 3-season use, I prefer a Pertex Quantum bivy with a Pertex Quantum shelled bag most of the time. In conditions where external moisture clearly overwhelms the concern for internal condensation, an epic bivy would be even better. I also prefer open tarps, which in terms of external moisture makes a big difference in my decision too.
A damp bag is never the lighter bag, esp since it takes so much longer to dry (IF you can get it to dry) than a wet bivy. Also the number of layers that moisture has to escape through is not as much of an issue as the breathability of each layer, and the temperature gradients which the consecutive layers create.
Sorry, this is a quick post, necessarily simplified, but hopefully concise enough to be understandable. If this is confusing, you’d be better off considering the climate and conditions you most often hike in, as well as the length of trip, rather than considering the abstract merits of two similar approaches.
