Three trade modules have a unique ability that no other module types have. Shipment Drone passively adds the bonus from Shipment Computer and Trade Burst, meaning it makes sense to level all three modules for their accumulative bonus.
I think this is a very interesting aspect and would like to see it in other module types as well. Examples for Mining:
- Hydrogen Upload (a very unpopular module now) could have a "Research Bonus Added: x%" that works with Artifact Boost
- Blast Drone could add a percentage of damage from Hydrogen Rocket. They're basically mini heat-seekers anyway so adding missile damage makes sense.
- Remote Mining (which doesn't really need levelling past 12) could slightly increase Hydrogen Replicator amount. Who doesn't like more hydrogen?
This gives passive incentive to level a variety of different modules while still feeling rewarding to the player.
This might require an adjustment to have all modules equipped on a ship in order to gain their accumulative passive abilities. Meaning you only get the +1200% trade bonus when Drone/Comp/Burst are all equipped on the same ship. Same goes for the Mining examples above.