Maybe in aikido, as I've never done it, but in the martial art I practiced for a while uke was feedback. The attacker sets the tempo for the technique, reacts to the tori's strikes, and while they're not supposed to apply the movement for them - they can lead the tori in the correct movement and suggest openings by being strong in some areas and weak in others.
While sharpening a movement, after both sides have some level of skill, they can provide feedback as to weak points in the technique - "I can stand up here, you don't have enough pressure or you haven't moved me off balance enough".
None of this feedback has to be verbal, but you can learn the techniques on both sides. Similarly, while people should learn from small failures, you don't want a catastrophic failure where you go and break uke's arm because someone didn't tell you that you were doing the wrong technique.