these slots where restricted for the very same reason you now want to remove that restriction.The way I see it there is no reason to not allow AFMUs, cargo racks, fuel tanks, FSD boosters, and really anything else that has 0 weight/mass. Such a simple alteration to make so many more ships more viable in so many more applications. One could even make an argument for allowing economy cabins. Just the visual of shoving people on top of one another in multi bunk hammocks or just taping them to the walls makes me giggle.
This literally takes nothing away from anyone and only gives more options to those who want to do something not combat orientated.
they where added to give a boost to ships that was meant to be combat focused ships, that is why not all ship has them... and general purpose ships outperformed these ships, because they had more internal slots, and thus could fit more defensive modules compared the combat focused ships, making the combat focused ships a a worse choice for their intended role.
These ships can still be used todo just about anything, but they are by design combat focused ships, so they are not going to be as efficient in serving other roles as those other ships are.