Is a truly adaptive, learning AI actually useful in game design, or is it a waste of resources?
It depends on the goal design of the game. If it's power fantasy, I believe dumb and slightly smart NPCs would be better to convey that.
If it's challenge, smarter NPCs could answer that.
The "Goldfish" vs. Narrative Balance:
I think the answer depends on how AI is learning or adapting if you will. If it's adapting while adhering to defined personality, it should be okay.