Why was it if I can ask ?
Let me roughly cite the argument here, (and sorry for the slightly sarcastic tone, please take it with a grain of salt):
Q: Why can't we have amount values in AO map slots?
A: Because it clutters the UI and can be easily achieved in the output options of the nested maps.
Q: But that would require me to go into the map tree which is cumbersome and I can't use these maps as instances as a change in output would automatically apply to all instanced maps.
A: To keep them instanced you can nest them into an Output map then and use the controls of the Output map instead...
Q: Still way more complicated than to just set a value in the AO map directly. Also, I always need to go through the map tree only to see what their output values are.
A: Whatever, we will not implement it, it clutters the UI.
In short, it's too advanced and will confuse the user.