I think the saucer separation ability was an example of theoretical thinking. The Galaxy class designers clearly intended a use for it. In dangerous times, the saucer would separate leaving the stardrive section free to act without it.

But, via practical real world experience, Starfleet learned that separating the saucer section was seldom worth it. It was too slow, where danger tends to occurred suddenly, and, even when there was extra time, Galaxy captains learnt the extra power generation of the saucer proved more useful than the decrease in mass. I doubt the War Galaxys from the Dominion War even had the ability and, if they did, it was only because it would take too much work to remove.

It's possible that the saucer section worked better as a one-use 'super-lifepod' but the original plan of combining and transforming ships clearly didn't pan out. Same thing seems to have happened with the Prometheus with Starfleet deciding the 'wolf pack' attack mode innovations, while effective, are better implemented via separate ships. Like we see a trio of Texas class ships doing to take down a Sovereign.

(There's clearly some guy in the Starfleet bureau of ship design who keeps trying to make combining/separating ships happen and keeps getting disappointed when they don't pan out in practice.)