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.