i'd have a subsonic bar as well, because the obvious bar that will get retrofit onto a bike you can't otherwise get low enough is this bar,
If you're fitting on a fit bike you don't really need the subsonic unless you hit the bottom limit of the Y adjustment. The X options for subsonic are same as Supersonic and Z very similar, so you go through your fit and then allow for the ~19mm stack reduction of the subsonic if you're hitting limits of the desired frame.
of course that's true. if your fit bike doesn't adjust low enough to hit what you need to hit, the fault is in the bike not the bars.
that said, from a retail perspective, i think it's always nice to demo the ergonomics of the actual bar you'll be riding. hence having that bar available for a fit bike demo. but then, that assumes you'll need the subsonic, which you won't know until the fit is over. unless you're pretty sure you'll need it before the fit starts, because of the nature of the customer and his problem. or, if that bar is what's coming OE on the bike the user aspires to purchase (such as the Canyon).
however, on simply a math level, yes, you're of course right.