The space between uke and nage is undifferentiated potential. If uke has lethal intent, he is seeking to apply some destructive force onto nage. Nage is maintaining aiki, that is, manifesting the natural complement to whatever uke's actions (and forces) are. There is an infinite set of possible ways for uke to manifest his attack, and the simplest and most readily available ones are strikes. Complex throws, takedowns or joint manipulations are possible, but their probability is much lower than the multitude of possible simple strikes. In the course of the interaction, the realm of superposed possible states is winnowed down to actual, manifested actions. (This is the transition from wuji to taiji.) Any strike that has not been made impossible, difficult, or futile should remain as the next most probable state that can arise. This is the natural unfolding of the situation. That is honest and natural, and it is the means by which aiki readily manifests in different forms, be it weapons, striking, grappling, etc. A dojin should be able to strike just as much as he should be able to do ikkyo. The reason ikkyo comes out (instead of striking) is that uke and nage mutually constrain the possible states such that strikes are being prevented and countered. This is done through the application of force and body positioning, and retained/regained balance, so yes, uke must be trying to affect nage's whole body. If he doesn't, we are doing a kind of striking practice (if we are being honest and simple). That is, it is uke's attempt at constraint of freedom that gives rise to something other than striking. The corollary is that a serious study of striking must be done by all aikido practitioners for this to make sense. An understanding of striking (and potential striking) is that really in reality from an Aiki viewpoint, striking and grappling are the same thing - grappling is "sustained" or tonic striking, and striking is sudden, sparse grappling.