Does anyone have any opinions on the Damn Secondary Hull Chasm?
I'd just like to point out that we don't exactly know whether it was there during the transwarp experiments yet. Since there are no stern views showing the hollow until ST6:TUC, we might speculate that the secondary hull was built "larger than it really need be" in order to accommodate machinery that was removed after the first few movies - either simply because the testing phase was over, or because the machinery had been revealed a failure.
Series production of ships with this now-useless cavity might still commence; plenty of real-world examples of such exist, such as armored vehicles designed
to take useless and indeed highly detrimental lead ballast merely because an early model was supposed to accommodate a wireless or an autoloader in the space!
The photographic model features an interesting greeblie in there, but this is virtually never seen. NCC-2000 features a blue "forcefield" glow, but this is unique to that individual vessel. And then there are the differences introduced in the "Flashback" model and the CGI models. Plenty enough excuse to think that the class was constantly being tinkered with, and that engineers well saw the utility of a "useless" expansion space.