the extra bulk, wt and cost. Not for this area. I'ts not that cold here, that often. I'll take an extra 1-2 of the 55 gallon drum liners instead. The regular size trifecta 2go system bivvy is much too small for me, so it does into the wife's BOB. We can zip them together and share body heat, which is worth 20F degrees or so. The extra bags are more versatile than the bivvy. It only costs me about 10F degrees of warmth and I can make that up by stuffing the liners with debris, getting inside of them and taping their mouths.