Since the pre-placed units are outside of Zeus control, they completely break the immersion and game logic since Zeus can't make them react to the fluid nature of Zeus-moderated missions. Lack of this ability makes mission creation far more tedious than necessary and limits Zeus's functionality.
While you are at it, you could also implement Zeus in the editor, for a powerful 3D editor that would make mission making 10x easier and better. Why this wasn't done so from the get go is beyond me!
Mission makers already have the tools to allow zeus to edit pre-placed objects so I don't really see what you're asking for here.
According to ticket: #0027155
Razazel says: "[...]Pre-placed units cannot be modified by the Zeus Game Master."
I really like the idea to edit pre-placed units, but I don't think that we will see that feature soon.
Honestly I am not a big fan of modifications. A core feature like Ares would be great.
I hope the developers will think and discuss about it again, create their own "Ares mod" as an official feature (like Zeus) and add it to the game.
While it is true that we already have tools to allow this via different methods:
it would be good to have a simple solution to this. Having a simple check box within the Add Editable Objects (ModuleCuratorAdd EditableObjects) to add all mission placed objects would be invaluable. While syncing objects with this module is trivial in small missions it is almost impossible in larger projects.
Note I am only thinking in terms of simpler missions where all objects start already placed on the map. If you're using other scripted solutions for projects to place dynamic units then the scripted solutions to Zeus compatibility would naturally fit better in the scripted solution.