A simple explanation for why we study Aiki ken saburi is to understand and develop the 3 main principles of Aikido so that they are the same with or with out ken. 1. Hanmi, 2. Awase and 3. Kokyu. In Iwama these are the 3 principles that make up Traditional Aikido and Saburi is one of the best ways to understand them.
Ken Saburi
http://www.youtube.com/watch?v=y6UjPDsdPso