read George Sensei's article: http://www.aikiweb.com/columns/gledyard/2004_08.html
Atemi is a fundamental aspect of Aikido. Taking a person off balance is a necessity, not an option. Atemi achieves that, whether or not contact has been made. A balanced attacker is a bad beginning in which to execute a technique. A good fighter will not get off balance unless YOU take away the fighter's balance in some manner or form.
Imbalance is the greatest moment of opportunity for training. The model of aikido being 'restoring balance'. Of course we have to create this construct to work it. It is in the benefit of both uke and nage to witness and learn the elements of imbalance. It is best to take it outside of human fighting dimension and to think of it as structural or universal' pieces'. The atemi is not you hitting him (or her) it is a meteor crashing toward the planet drawn in by a vacume.It may also be a black hole. Etc. This concept will give you more tools to work this game.