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.
Here's a thought. The attacker, by virtue of attacking , is already off balance. We are in the business of early detection and exaggereation. How quickly can you perceive the imbalance, and how centered are you when you do? Then, go from there into the appropriate spiral for magnification.
Ow, says the bug, it's hot under here.