I guess in case of low jump tech its more useful to mount the mil. ships with their own Jumpdrives instead of using Jump tenders?
Well, you can do this. Of course, at low jump tech, the jumpengine will also take up a large part of the ship, leaving that much less for arms and armor. Personally
Seconded. Especially at low tech (because the efficiencies are so much worse), Aurora pushes you in the direction of specialized ship designs. The reason is that, in the abstract, the cost of a ship can be broken into "payload" (useful stuff like sensors, missile weapons, jump engine, etc. that accomplish a job) and "support systems" (non-task-accomplishing stuff like armor, engines, bridge, etc.). At a particular tech level and level of capability (armor level/speed), the ratio of support systems to payload will tend to be about the same, independent of the payload. In Apollo terms, the payload can be thought of as the mission modules at the top, while the "support systems" can be thought of as the Saturn V - as you can see, in this case there's a huge overhead of support costs for every ton of payload.
The problem with a general purpose ship is that you can usually use only one type of payload at a time. Jump engines only work at jump points. GeoSurvey modules only work at bodies. GravSurvey modules only work at grav survey points. So if you cram all three of these into the same ship, you'll be paying the same support burden as if they were in three separate ships, but the payload will only be 1/3 as effective because only one part of it can function at a time.
So if you put jump engines into a military ship, then the "jump engine" payload isn't doing anything while the ship is fighting a battle, and your overall fleet is less powerful than if you'd split the functions.
John