Got it in one. At some point it becomes cheaper to ferry the dropships rather than have each individually powering itself from point to point. Get it to system X and then have the core disabled and presto! you now have a big ferry for jump point to in-system locations.
What you gain in fuel savings comes at a cost of strategic mobility- most warships are slower then then slowest dropships- with few exceptions most of the older retired dropships have a thrust of 2/3 or 3/5. Most dropship's acceleration starts at 3/5 and assault dropships have thrusts of up to 8/12.
Which doesn't mean much if you're looking to make a planetary assault and have your troops in fighting trim. There is a reason why most transport droppers enjoy a low thrust in comparison to their assault counterparts. A High G in-system burn, while reducing transit time, is physically punishing. The human can't sustain High Gs for long periods of time and not suffer the consequences - heart attacks, fatigue, etc.
This is one of the reasons why large assault forces rarely use a pirate point for jumps. True, pirate points are risky and cut down transit time considerably, but when your assault force reaches a certain size, it's all about reaching the target world in one piece and well rested. (There's something to say about organization.) Given the relative time it takes to establish aerospace superiority, select landing zones, etc., (depending on the world) having a powerful in-system ferry, (that basically equates to a monitor) strategic mobility is a moot point since the WarShip becomes a powerful tactical asset for the defender.
While basic duties would certainly save wear and tear on transported Droppers, the ferry's real benefit comes from the ship's tactical capabilities. A powerful WarShip is still a powerful WarShip, whether an attacking force uses a standard jump point or not. Either way, unless the attacker is working a quick smash and grab and times the raid to coincide with the WarShip being out of orbit, an occupation force will ultimately have to contend with its presence and all of the tactical possibilities having a WarShip entails - orbital bombardment, capital weapons, etc.
At that point, it doesn't matter how long it takes the WarShip to reach orbit or the lack of strategic mobility. It ultimately falls to use and how/where you envision the ferry falling into your military organization and defensive doctrine.
