In following the discussion I think there is a parallel between training to move the body properly and without tension when faced with static grabs and another learning principle found in Iaido and other sword arts.
As I and others have said earlier the static work is important to ensure proper structural alignment, power generation and other elements that are important to Aikido technique.
A parallel I have experienced in my own training with the sword also lends to this sort of training. One can do thousands of suburi or kata and work on ones cutting form with no resistance (cutting empty air) for a long time, however one experience of tameshigiri practice will automatically reveal weaknesses in structure, cutting form, power generation ability etc. much like Aikido static practice.
In tameshigiri/sword arts the quality of your cutting ability (and by extention your non-resistant training in form) is demonstrated by the quality and success of your cut. If your understanding and execution are poor then the cut will be terrible if at all successful. In Aiki waza the quality of your movement, structural patterns, technique etc. are all demonstrated by ones ability to effect kuzushi on ones partner without become unusually tense and using excessive upper body muscular strength. Again, if understanding and execution is poor ones partner will not have his balance broken or alternatively one will be resorting to excessive tension, upper body strength or some other method to compensate for what is lacking in the skills being measured by the practice.
Just a few thoughts. Great comments by Ledyard Sensei and others.
LC
