from Principles of Aikido, by Mitsugi Saotome

Ukemi is the art of being uke, and the quality of nage's practice depends on how well uke has learned this art. Ukemi involves creating the conditions that make a given technique appropriate, responding correctly to nage's movements, and taking whatever fall concludes the technique. In short, uke is responsible for creating the conditions that allow nage to learn. If uke has no sense of the effects of a technique, no resilience, or no responsiveness to nage's movements or if he is fearful or awkward at falling, nage will not be able to study the technique effectively.

In practicing any technique, partners will alternate taking the roles of nage and uke. You must not regard the time that you spend in the role of uke as merely marking time between your turns at being nage but as a learning opportunity of importance equal to or greater than the time you spend in the role of nage. In fact, those who excel at taking ukemi will most likely excel in technique also, for they will be able to absorb knowledge through their bodies of how a properly executed technique feels, as well as absorbing knowledge through their minds. Developing good ukemi is the shortest path to acquiring skill in Aikido.