Good post Chris. One question...if someone can extend the power of their body out to their own limbs...could that change the nature of your equation?
Well I'm not really sure where you're going here, but I'd say no...
Proper body use does enable you to move your body weight and larger muscle groups (legs and back) through your arms. Making your arms more powerful. But this isn't an advantage for the grabber (uke in Aikido techniques). What he wants to do is to grab and control the center of the other guy. More links in a chain makes this harder ( the other guys elbow and shoulder). It's much simpler, and more powerful to directly grab the body. However if they are armed doing that will get you killed.
Now for the guy being grabbed it can be an advantage, But also a disadvantage. The guy being grabbed wants to stay as flexible as possible. This makes it harder for the grabber (uke) to control your center. But being able to suddenly use the power of the whole body through the limbs can make escaping much easier.