The Galaxy class starship was designed with the ability to separate the saucer from the stardrive section, so that the “floating city” part of the ship could be left somewhere safe while the rest of the ship galavants off to do something risky. We see this happen precisely once, in the season one episode Arsenal of Freedom. We also see saucer separation deployed for a handful of tactical and or emergency uses (such as against the Borg in The Best of Both Worlds, or to escape the breaching warp core in Generations).
So, this seems like a useful ability to have, and the Enterprise is constantly being sent into dangerous situations. Why not use this ability more frequently?
It’s not just the lack of warp that presents a problem. It’s the lack of power generation. If I remember correctly, and I’ll admit I may be misremembering, but isn’t the bulk of the Ent-D’s energy generation done by the warp core as well? So if the saucer is separated, any heavy power use (i.e. the main phaser array, heavy transporter use, etc.) is going to require the saucer to eventually link back up to refresh it’s power reserves lest they end up in a Voyager power rationing situation.
Edit: This same point is what drives me crazy when I see the separated warp nacelles in DSC. If the ship loses power for any reason, the nacelles are going to float away which just adds to the list of problems to solve when getting the ship back under way. Also, if there’s a momentary power interruption to the beam emitters holding the nacelles in place, what’s to stop them from launching off like warp-powered torpedoes?
To your comments about the floating nacelles - what’s even supposed to be the point? I can’t imagine how having nacelles detached offers that much of an advantage.