Have you noticed that EVERY ship has an astogator? And in a lot of cases an assistant? Home Fleet at the Battle of Manticore had something like 300 capital ships meaning at least 600 of them just waiting in the system.
Their STARSHIPS, you should expect starships to have at least one astrogator because you should expect them to be moved someplace out of a system at any time. Or in-system....its a navigation post.
Ok, ships assigned to Home Fleet are likely to remain in the Manticore system until assigned elcewhere but they do need to manuver and move around within the system and that includes both between the binary stars and probably out to at least the Junction at need.
This is a position (and skill) that needs to be taught and learned plus experienc gained. If you have 300 capital starships you need at least 300 astrogator/navigators for them. In a place like the binary system with the Junction and then all those termini connected you need the ability to take your ship(s) safely and quickly from point to point with accuracy. And when you get where you are going you have to be able to go....anywhere.
It is not unreasonable to put trainee astrogators on duty with Home Fleet ships to gain practical experience before assigning them to smaller ships (in whatever capasity) to gain more experience.
Heck, having to figure getting SAFELY between the hyper-limits of the binary system stars is a non-trivial exercise. This is RMN we are talking about. They have to anticipate needing to move ships between elements of the home system and out to at least the Junction if a problem comes up. So every ship has to have at least one person who can do the job (and train others) to do that. I don't see being the Admiral assigned to an SD and being told the ship is being transfered to Spindle or some system out in Silesia from Manticore saying "I don't have anyone qualified to do the navigation so we can't comply till you send us someone who can handle the job" Big Smile.