Uke's role is to help train nage in the art under examination. Uke, at a base level, should not resist a technique or a throwing exercise. They should require that their balance be broken, with proper application of technique. If the appropriate limb is locked, and balance is broken, then you are not "taking a fall" by not resisting. If the techniques has to be performed at 10% of speed to learn how to break balance then so be it. It is a simple thing to resist if you know what is coming or provide no energy.
When working with lower kyus, doing drill work, and working a new technique, uke is required to help nage get the feel of the throw. Resisting by going limp, not entering, locking down, providing random energy in the wrong direction all constitute barriers to learning; they are ego boosts to a misdirected uke.
That being said, dan ranks should be held to a more applicable art. Strikes that track uke, punches that retract quickly, getting swatted with uke's free hand (improper location), holding upon landing to test stability, getting kicked in the butt when standing in the wrong place after a throw are all valid training modes for yudansha (as well as upper kyus). They should occur with agreement and knowledge of the participants and more typically are part of a dojo culture as defined by the chief instructor. Introducing reality to the upper ranks is important after all if the advanced students cannot effectively utilize a technique unless attacked just so, then what is the point, peaceful as it may be Aikido is a martial art not dance style. Reality is important, but has the right time and place in the learning process; overzealous green belts are not the right time and place.