Uke shouldn't move unless moved; ukemi really should be an attempt to regain a stable posture from which to fight back.
I think I see what you mean, but these two statements are contradictory. At least in practice, an uke who refuses to move may be stable, but usually is not in a position from which they can fight back.
The classic "immovable uke" -- someone who clamps down on your wrist and grounds out -- does not strike me as particularly helpful, either for training against "real" attacks or for actually learning anything about aikido. Even (perhaps especially) in static practice, uke needs to be sensitive to what nage is doing and respond in a way appropriate to nage's skill level.