I got using a half bag, pied d' elephant, when I was actively climbing all season. To me it was 1 step above stuffing my legs into my pack for the night. I still like them and I still use one but I think from a weight standpoint a lt. jacket and a lt. bag will come in under the half bag and heavier jacket you will need. When climbing I was always hauling a bobp anyway [a big ol' belay parka].
That said, wild things makes a synthetic insulation with epic and nunatak makes the nicest down half bag I've ever seen. Half bags work under the same type of protection you would use a sleeping bag with, but it is easier to spend the night in a sitting position [as in ledge] in a half bag.