Even if the AI is friendly being able to predict their behavior is still desirable. Nothing worse than friendly AI ruining your game because it went off and did something you couldn't anticipate.

What if the friendly AI called out it's plan, and the player then could have an agree/disagree option with the proposed strategy?

If done well, it could feel much more like you're playing with a human partner.

Then, sure, because it is now predictable. Alternatively, I don't think it would be an issue if the AI was good enough that it could do unpredictable things that were actually intelligent (which would pleasantly surprise the player) or if the unpredictability and occasional awfulness of the AI were somehow an intentional challenge of the game.

If it's too smart on your side, wouldn't it do all the playing for you? For example, you enter a room full of enemies and your AI partner kills them all the second they come into view. What's the point of playing, then?

True - but that would require that would require that the AI partner is both overpowered (they can kill the whole room without risking to take damage) and completely ignores me.

maybe an alternative appraoch could be to have an AI that tries to figure out what you're trying to do - and then help you with it.

E.g., if the two of you have frequently witnessed a certain enemy launching an attack that can only be deflected by a particular shield spell, an AI healer could "learn" to cast that spell in advance when they see such an enemy.