Since the bottom half of the jacket is generally buttoned-up and doubled-up flat over itself, wouldn't it be true that canvassing and structure is less important there? In fact, wouldn't one not want canvassing there so as to reduce bulk? I feel like full-canvassing is more critical in single-breasteds especially when they're unbuttoned, but this is still a mysterious topic to me.
This is a complex issue, and while I'm marginally ignorant concerning the detailed intricacies of tailoring, I suspect there is no yes/no answer to your question. Fully canvassed tailoring, as does fused tailoring, and so-called un-constructed tailoring, each come in many versions. For example, both fully canvased and fused construction actually use
canvassing, and such canvassing comes in variety of thickness and stiffness. Added to that is the preference of the wearer as to how he structured he wishes his clothing to appear.
But as a generality, all things being equal (Which, of course, they never are.) full canvasing can achieve both better drape, and more shape. than an equivalent fully fused jacket. And rather than believe it's less significant on a DB, such advantage might be even more so, since a DB will cosset the chest to a degree that a SB really doesn't, which I would think should make both drape and shape even more important.