You mean that SF constructs another Galaxy class saucer section that would hold weapons and nothing else?
Specifically in times of war?
The Federeation doesn't have that many wars really.
They are more peace oriented and building a dedicated weapons platform that would be held for times of war ... seems a bit impractical.
Most of the time, they wouldn't be utilized to begin with and it would be viewed as a waste of resources really.
I think it would be more practical to make another re-fit of the Galaxy class star-ship by adding warp capability (another warp core and nacelles) to the saucer section along with 2 more additional torpedo tubes.
Adding more torpedoes to the ship is easy and storing them on a vessel such a Galaxy class saucer is not that problematic.
They can always convert several crew quarters as torpedo storage areas, connect them to the torpedo tubes and you essentially get another 250 torpedoes for the saucer section.
They could also add several deploy-able pulse-phasers to the mix (but not too much).
It could be done if they wanted to make it slightly more combat ready.
The Venture type refit (nacelles receiving phaser-strips on the top section) coupled with some minor interior changes to accommodate more torpedoes and another warp core into the saucer section sounds more than enough for the Galaxy class.
And when separated, they sections become their own ship so they can easily engage multiple targets and divide their firepower in different directions.