Ukemi is a critical skill in aikido.  In a sense, ukemi is more important than anything else in learning aikido.

Initially, ukemi is important to avoid getting hurt.  We fall down a lot in aikido, and, especially on a soft surface, it is easy to learn to avoid getting hurt, even if you are big, and even if you are clumsy and out of shape.  As we get better, often we get thrown harder.  Our ukemi needs to progress so that we still do not get hurt.  The goal is for a young fit aikidoka to be able to take being thrown as hard as anybody can throw them, and still not get hurt.

Ukemi can be useful if we fall outside.  One local aikidoka fell three stories onto concrete at a construction site, rolled to his feet, and walked away uninjured.  We don’t have as much ice as up north, but there are still lots of opportunities to avoid getting hurt by using ukemi.

As we progress, it is an important part of ukemi to stay connected to nage.  If we grab an arm and the person can pull free and hit us, this is poor ukemi.  We should be able to move to absorb the energy and prevent him pulling free.  This might mean we have to move our body.  But it is better to move our bodies and maybe roll away, rather than be stiff, stand where we are, lose our grip and get hit, and then hit again, and again.

This logic means it is important to get space and roll back to our feet facing nage.  Crumpling to the ground at nage’s feet is not too bad in the dojo, but aikido is a martial art, and crumpling at an attacker’s feet outside the dojo is likely to mean we will get kicked in the ribs, in the head…  Stiffening up to stop somebody throwing us is not a good idea either, as that allows nage to put tremendous force onto part of our body, such as a wrist.  Resisting technique is probably the commonest cause of injuries in aikido.

Moving, blending with the technique is important for uke, and ultimately it is critical to effective performance of kaeshi waza (counter techniques).  The same stickiness and flowing ukemi that is important for surviving also lets us take the initiative back from our partner.  Uke can become nage, and if this is done well, can throw his partner.  If his partner also has good ukemi, that partner might also be able to flow well enough to take control of the flow back from the new nage and reverse the kaeshiwaza, and this can go on until one person gets sufficient control to finish a technique.  This is really advanced aikido.