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?
I honestly do not know why they chose to do that. I can't think of any advantage to it at all.