The reasonable hypothesis is that the Galaxies seen during the Dominion War were fully equipped and constructed.
But supposedly not even the E-D was ever fully equipped or constructed when operating in TNG. A mere thousand people were rattling about in the nearly empty corridors, and even if every one of them had a room of their own for whatever work they were doing, the ship would feature unused spaces...
Probably ships sent to battle would have all or most of their combat systems installed, even if they lacked 90% of their research gear and 50% of their medical facilities. But even that isn't a given. Many a ship was sent to battle in WWII only partially armed, because having something out there was better than having nothing. And if you have a big ship with complete shields but only one-third the armament, you still present the enemy with a full target he has to cope with.
We were told, basically by Worf in "Heart of Glory", that the Galaxy fights better separated than joined. We were not told, however, that a separated Galaxy would fight better than an average combat starship. And we were soon shown that a ship shaped almost exactly like Galaxy, namely the Nebula, would fight just fine when not separated.
So was Worf wrong? Quite possibly. The separation feature wouldn't be there in order to make the battle section more badass: it would be there to send half the ship to safety while the other half fought. As said, this would mean compromises, and quite possibly the saucer would take away important features when leaving the battlefield. If there were no civilians to evacuate (as there would never be in a ship deliberately sent to battle - there could be civilians, but only of the sort that were supposed to see battle), then it might well make sense to retain those important features.
I for one would hate to go to battle without those big nasty phaser strips on the saucer. Or the two powerful-looking impulse engines.
Timo Saloniemi