SakashiroRav3nkinSo, if its not to repair each other then what is it for? I'm not trying to be "that guy", I really want to know your logic. Kurakil's comment is about where I stand on this as well.
Jet cone boosting damages the frame shift drive, so if you want to use the neutron highway on a regular basis, you need an AFMU to occasionally repair the accumulated damage and prevent malfunctions. Stacking AFMUs increases the total number of FSD repairs you can do before you have to either return to a station, synthesize repair ammo from raw materials, or stop using the neutron highway. They basically serve as ammo storage. AFMUs are weightless and do not reduce jump range, so it makes perfect sense to put an AFMU into each slot that isn't used by anything else. With those four AFMUs the DBX has a total repair capacity of 10,100, i.e. more than a class 7 AFMU. Once they are all depleted, you synthesize ammo only for the largest one, so that you get the most mileage out of your materials.
You don't need multiple AFMUs to repair each other, because even in the unlikely event that your single AFMU is destroyed, rebooting the ship will restore just enough module integrity to make it operational again.
Ok. I know all of this, so you must intend to be out of touch for quite a while if you feel you need that many AFMU repairs. Good luck to you then. Maybe I'll see you out in the void.