They were sorely short on ships in Redemption to span their tachyon detection grid. Had the ent-D separated, they would have had at least one more ship , though perhaps there was a good reason not to do so (i.e. the need to fight, or to pursue at high warp, or perhaps the saucer section couldn't even generate /maintain the tachyon grid)
Here it's remarkable that high warp performance was for some reason vital for simply maintaining the grid. That is, the heroes specifically fake a warp drive malfunction on one of the ships to create the opening into which they could lure the Romulans. This ship is to "fall back" for being unable to play her role in maintaining the grid, which in turn means the ship pretends to still remain capable of moderate warp, or else falling back could not physically happen, not there in the middle of nowhere.
So the saucer would need not just a working and sustainable warp drive, but one with performance comparable to a fully functioning starship rather than a slightly damaged one.
(That the saucer has a working and sustainable warp drive is not in much question, considering "Encounter at Farpoint". That this saucer drive furthermore is capable of accelerating to warp all on its own after separation is in turn established in "Arsenal of Freedom" where, if this capacity were absent, LaForge ought to be hanged, drawn and quartered before being shot while keelhauled when he first separates at sublight, then tells the saucer to fly to another star system...)
Separating would help basically whenever the heroes need to divide their assets, sometimes even if we assumed the saucer for some reason were unable to go to warp that particular day. Yet in most cases it would be overkill. Why not simply leave a shuttle or ten to support the away team while the whole ship warps away to another assignment? That alone would make a world of difference in situations like "Final Mission", where nothing initially suggests half a starship would be needed, yet OTOH it would cost nothing much to apply a contingent of shuttles as backup (even if Picard agrees to fly on Dirgo's shuttle and the Starfleet teams agree to maintain their distance until needed).
Timo Saloniemi