It's not because you don't use it that it doesn't exists :p
UID is a system data used by Construct, you can't change the main way it works.
Your test shows objects are always created/destroyed (when you drag them on the floor, when you take them back)
Of course if you think with a minimalist view, it's okay to do that.
Now think that i'm working on a simple graphic game, 20 levels, 800 objects. If you do a RPG how many things will be added/destroyed all the time ? Yes, much more than 800.
If you use Background to draw your inventory boxes + your objects it wont take that much memory.
There is a memory limit to create and destroy all the time, that's why i'm talking about it and asking you.
Of course it's millions of objects, but the limit exists. It's the size of an Int (Google it if you need it).