The writers sort of used up their dialogue options in "Heart of Glory" where Riker wonders whether the saucer should be separated and Picard says it's too soon as they don't know yet what they are facing. That basically covers all the angles: it's always going to be too early or too late, and there's little point discussing the latter case because it will only make the heroes look bad.
Timo Saloniemi
Timo mentions an interchange between Picard and Riker at the very beginning of "Heart of Glory":
WORF: Captain! Communication from Starfleet! They have reported a disturbance in the Neutral Zone.
PICARD: Of what nature?
WORF: A battle! Quadrant nine, coordinates zero seven zero, mark three. There's no information who is involved. They are asking if we can investigate.
PICARD: Answer affirmative. Lieutenant La Forge, lay in the course.
LAFORGE: Aye, sir.
DATA: I have no reports of any Federation ships in that area, sir.
RIKER: Ferengi?
PICARD: It's a little out of their territory.
RIKER: Shall we separate the saucer?
PICARD: Let's get a little more information first.
LAFORGE: Course set, Captain.
PICARD: Speed, warp seven.
LAFORGE: Aye, sir, warp seven.
PICARD: Engage!
(A short flight later)
LAFORGE: We are now approaching the Neutral Zone.
The episode apparently takes place in close proximity, if not within, the Romulan Neutral Zone. If there had been Romulan activity nearby, a saucer jettison would free up the stardrive section to investigate and deal with it, but it would also make the saucer vulnerable if it should happen upon other hostile starships.
Still, the brief interchange between Riker and Picard seemed to just be more of an aside than a serious deliberation about actually separating. FWIW, I always took the notion of saucer separation to be a contingency that was situation-specific, not something they would automatically do.
Later on, when the Enterprise-D Bridge crew discovers the approach of a Klingon cruiser and hails them, there is this exchange:
(On the Bridge)
PICARD: Lieutenant Yar, where are they now?
TASHA: They're with Worf on deck seventeen.
PICARD: Deck seventeen?
TASHA: Yes, sir. Near the auxiliary turbolift to the battle bridge. Shall I alert Lieutenant Worf?
PICARD: No. Send a security team.
RIKER: You don't think Worf would allow them access to the battle bridge?
PICARD: I think, Number One, we cannot assume anything.
TASHA: Captain, shall I stay at my tactical position or lead the security team?
PICARD: Lead the security team, and, Lieutenant, you understand with whom you are dealing.
TASHA: Aye, sir.
(In one of the Enterprise-D's corridors...)
KORRIS: We have heard this ship can separate in time of battle.
WORF: Yes. When relieved of its bulk, the Enterprise becomes an exceptional weapon.
This, to me, suggests a couple of things:
First, it suggests that saucer separation is used as a military maneuver to allow the saucer to proactively separate safely away from a dangerous venue so that the stardrive section can preform whatever aggressive action is needed without disturbing operations within the saucer-hull.
Second, the notion of "bulk" comes into play. Apparently, Worf is suggesting that saucer separation relieves the stardrive section of "bulk", meaning the "bulk" of a Galaxy-class starship is in the saucer, not the stardrive section of the ship. (This plainly contradicts what Sternbach and Okuda asserted in the TNG Writer's Technical Manual, and later in their published TNG Tech Manual, about the stardrive section housing a huge volume of deuterium and antimatter for warp drive fuel.)
Also consider this exchange from "Heart of Glory":
(Enterprise-D's Main Engineering, Dilithium Crystal Chamber)
KORRIS: Brother, I knew you would come! (Worf climbs the ladder) Now I, we have a chance! I could not do it alone, but I would rather die here, than let the traitors of Kling pick the meat from my bones! With you it will work.
WORF: What will work?
KORRIS: I will demand Captain Picard give us access to the battle bridge. We will separate from the rest of the ship, then together we will light up the galaxy! Imagine the fear which will roll before us!
WORF: Captain Picard will not comply.
I quote this passage because it seems to reinforce the previous quote. Taken separately, it doesn't say much. But taken in the context of the episode and what Worf told Korris earlier, it seems to reinforce the notion that separation liberates the stardrive section to preform military tasks.
Now, consider this scene from the following episode, "Arsenal of Freedom":
(Logan enters the Main Bridge)
LAFORGE: We are getting out of here! Lieutenant Solis, set course three one five mark zero, zero seven!
SOLIS: Course set!
LOGAN: You are leaving them on the planet?!
LAFORGE: Speed warp five!
SOLIS: Warp five. Aye!
LAFORGE: Engage! Hold course and speed for twenty eight seconds, then come to a full stop. Mister Logan, had we stayed, we would have been destroyed. Now, there are over a thousand people on this ship. I have a responsibility to them.
LOGAN: What about your responsibility to Captain Picard and the members of the away team?
LAFORGE: I have a responsibility to them as well. Mister Logan, you are going to take command of the Saucer Section. Backup crew, report to the main Bridge.
LOGAN: You're going to separate?
LAFORGE: Yes, and I want you to take the saucer section and proceed immediately to Starbase one zero three.
LOGAN: You can't fight what you can't see, and you still won't be able to see that thing.
LAFORGE: Maybe we won't have to.
LOGAN: Risky.
LAFORGE: Yes. That's why we're going to separate. Worf, Solis, T'Su. Report to the Battle Bridge. I'll join you in a moment.
I added underlining for emphasis to show that LaForge's orders and rationale are designed to accomplish two things: (1: LaForge wants to remove the Enterprise-D from the battle scene before jettisoning the saucer; possibly outside of the Minosian star system... and (2: LaForge wants Logan to pilot the saucer to a starbase, just in case the Enterprise stardrive section does not succeed; LaForge is setting the saucer up to be a civilian escape vehicle.
There is one major flaw, though, in the notion of saucer separation: impulse power. Is a Galaxy-class saucer only capable of relatively low-speed maneuvers? Is it sublight-only? If so, how is Mr. Logan ever going to reach that starbase before the saucer's viability expires? The starship Defiant in DS9 shows us that Federation technology can build warp engines into an armored hull. If we assume that a Galaxy-class saucer actually has a pair of small warp nacelles built-into her saucer hull, then it is possible that Logan would be able to reach his base even if LaForge failed in his mission.
I bring this up because starships are supposed to be deep space vehicles. They fly, by definition, outside of their home space. Why would we not assume that a saucer-separation, under proper circumstances, would be a realistic strategy for a starship commander to employ while on the frontier?