It depends on the context which weapons you equip.
If you have two identical slots, in many cases in games like TFTD I most often equip them with the same, be it DUP or when playing some mods (like TWOTS) then some other weapons. The problem is that unless you have something like vanilla laser beam/sonic oscillator that has essentially unlimited ammo, you may run out of your ammo if you have just one gun of a particular type (if it was otherwise and DPS was not a concern, I might more often equip the craft with different weapons). For example in vanilla TFTD you need double DUP launchers to take down all the small subs with one craft reliably.
What I think you're essentially saying is that if everytime all the weapons on the craft are identical, there is not much difference between different attack modes (except if you want to use aggressive, for example, to make sure the USO does not flee) - and in that case the attack modes might be redefined to mean something else.
The problem with that would be that in many mods and games there are valid reasons that all the weapons are not the same, so you'll need some of those current attack mode features anyway. Also, there are major mods like XCF where crafts have different kinds of weapon slots, one for example for short-range cannons, the other(s) for long range missiles, etc. (also either/or, not necessarily 'and'). So what you equip might change a lot. And to avoid confusion the interface cannot change each time you engage.