The point of 'efficiency' is that:
- It's an extremely plausible thing to expect given enough raw cognitive power.
- It's a compact reply to people saying "An AI will do (believe) X because Y" when the AI is supposed to have a lot of cognitive power and a human could think of a better strategy (inference) for Y.
- It has understandable precedents in market prices and in superhuman game-playing.
- It's one of several compact properties that lets us distinguish important superintelligences from much weaker objects like corporations which people are tempted to shoehorn in as Objects of Equally Great Concern.
Efficiency is indeed a very powerful property and is often overpowered as an assumption for the conclusion being offered. On the other hand, it's also an important basic idea to understand because an AI which isn't efficient in any domain is not relevant (it must not even be able to play superhuman chess, if it's not efficient at chess).