Does Jus in Bello favor the big battalions?
You ask: "Does the application of international humanitarian law to asymmetric warfare give relative carte blanche to traditional armies? Does it matter?"
No, it doesn't give them a carte blanche at all, but I don't think that's what you really meant to say. Applying it consistently to both "traditional" large-scale state-based armies and to guerrilla forces would definitely give the first side a systematic relative advantage. This is not surprising. The principles of "just war" doctrine have always been intended to formalize and regulate warfare as closely as possible, on the assumption that war can't be done away with. As it has emerged, it is intended to favor formal armies and their characteristic war-fighting techniques, and in other ways it has this effect in practice. On the other hand, if jus in bello constraints are applied exclusively to formal armies and not to irregular forces, this has the opposite effect.
"Does it matter." Well, yes, in a lot of complex ways ... which I'm not foolish enough to try to address in a "comments" box..