"And drown in the condensation. ; )"
Well, since this thread was originally about bivy condensation before I took it off-topic, you've got a point. However, I was referring to a technique Ryan Jordan has successfully used, and spells out in his review of the ID eVent Overbag on this site:
"If you are using one of these bivies as a standalone shelter, the Overbag/Bivy gives you the option to flip the bivy over for additional weather protection in case of hard rain. I found that to be particularly advantageous in a few situations, even in the summer..."
I presume Ryan would sleep on his side or stomach the rest of the night to keep his mouth near the opening.
I would only consider this in an emergency situation, personally. I brought it up simply to point out one has other options if one's tarp were to fly away, or if you got caught in the middle of the night in an unexpected downpour without your tarp setup. You don't have this versatility with a pertex bivy, and since this is a worst case scenario, one would benefit from the tarp the other 99.9% of the time it doesn't blow away =). Also, I'd think the opening in the bottom of an inverted bivy would be as breathable, if not more, than the ID Chrysalis or the Nemo GoGo, both of which force you to breathe directly into the bivy.
I think perhaps the biggest advantage of the ID eVent bivies (besides their more robust design and use of eVent) is the beaked hoods that allow you to sit up in the bivy and still be protected from the rain. You wouldn't be able to do that in a Montbell. But again, I would only use the Montbell as part of a tarp/bivy combo.