I guess the choice of innards is symbolic of whether we think that the neck connects the two hulls, or separates them.
A maze of shafts, conduits and piping would act as a connecting element. A series of lounges would indicate space unused for connecting and thus shunted to some other use.
There are good reasons to argue either way. Since there are two hulls to begin with, Starfleet must be thinking in terms of making them separate - so a structure between them is likely to be a separator rather than a connector... Or perhaps rather Starfleet wants to operate two different sets of machinery and capabilities, neither of which can make do without the other, and thus needs to install a connecting piece that allows for the sharing of resources.
FWIW, surprisingly many starship designs feature this "bottleneck", indicating that Starfleet likes such choke points and doesn't want them any wider than absolutely necessary. The ability to separate the two hulls is quoted as a rationale for this, implicitly or explicitly. Again, one may argue that the separator is built without connections so that it would be better at separating - or that the separator exists in the first place because a connection is still needed and the two hulls cannot readily fly on their own even though Starfleet wants them to. But the extremes are unlikely to be the right answer, because we also see "intermediate" necks, such as the extremely thick Excelsior
one, negating many connecting or separating arguments.