With any game that's based on an IP, you have to take into account balance along with the IP canon. However, in the end balance reigns supreme. We do not have 10/12 weapons on cruisers because of balance issues. Loading 2-4 more weapons on a ship would be horrifically overpowered in STO.
[Unrepentant] Lapo@overlapo: the problem with space STF
is that you can't properly teabag your defeated opponent
Unrepentant: Home of the Rainbow Warrior and the Rainbow Brigade.
Been asked for before. Folks have even suggested making the weapons into a more flexible slotting system where different weapons equate into a different number of slots and you had to balance them yourself.
You may want to search the forums (Yes, that's a hint. Even canon as they used the search function on the shows.) for discussion about the pros and cons about this.