"A loadout of AMRAAMs in an otherwise less-than-usable space is a relatively small investment in the overall design and if they never get fired, that's OK, but they'd be a damn useful secondary."
I understand where you are coming from, but there is no such thing as "less-than-usable space" in modern aircraft design, especially one that fits a "box" more than 12 feet long and probably a third as wide and tall.
Take any cutaway of modern combat aircraft from 1950 onwards and there is no "empty space" anywhere. If you have a cubic foot available, someone will cram in avionics or systems.
Having to cater for an additional weapons bay, door actuators, extender trapeze, power, heavier empty weight (weapons bays are giant holes in your structures, which structural designers abhor) , plus making sure you have enough cg range, to live with and without the weapons aboard in every corner of the flight envelope and combination of remaining fuel/bombs, is a major PITA. If you get to the point where you have empty space, it's because you could have made the vehicle smaller/lighter/cheaper in the first place.