Charles Warren writes:

From a health standpoint it’s probably good for your bones. After a certain age we don’t preserve either muscle or bone we don’t use. From a martial point of view, ukemi is key to reversing techniques. If you don’t allow the original technique to progress pretty far along its line any kaeshi waza is going to be forced against its strength. And, finally, kaeshi waza is a stage in learning. Without natural kaeshi waza your perception of any situation will be limited. Beside, will you naysay O Sensei who is quoted as saying that ukemi is vital to learning?