Wouldn't slaping on a weapons platform that the ship was not designed to work with in the first place cause some problems? Not only from the standpoint of the stardrive and original saucer being designed to work best together, but also from a power supply standpoint.
I'm not sure this makes much sense, but couldn't the ship's power system develope some sort of "battery memory"? Like, the ship has worked with the same saucer eating up the same amount of power for x amount of years, enough time that the power system has adapted and is "comfortable" with supplying that said amount of power all the time. Adding a weapons platform that eats up loads more power would probably stress the current power system, and be massively inefficient. If it was a new-build Galaxy that was built with interaction with this platform in mind, that problem might not be present.
So, if these platforms were indeed to be built, maybe they would be designed with the "second wave" of Galaxys in mind, while still retaining the ability to be used with older Galaxys, but much more inefficiently, and with some possible problems.