I think that practicing with a compliant uke, refines technique in the hope that real ki will develop over time in nage.To learn rhythm. How can we learn otherwise? the first moment of the initial attack should be committed and strong, then uke follows nages lead and, like a mirror, uke teaches nage about the "technique".(by the action of his body)
I agree with Kevin: If nage is not really leading or throwing, then uke should not fall if he regains his balance. A fine line. The spirit of cooperation is needed.
In my opinion, Ki cannot root in a tense body. It stops the flow which hinders learning.
Every meditation/spiritual practice (that I have seen) stresses letting go of unnecessary tension. Aikido, I think, is not different.