Contrary to some people's belief, atemis in Aikido are not form of agression. Aikido is a multi-faceted martial art, wherein atemis CAN coexist with the application of non- agressive, i.e. "non- impact" techniques. Atemis do not need to be rationalized in the martial context of Aikido; the fact of the matter is, in Aikido, the atemi serves a vital purpose. It is not used to cave in an opponent's face, it is used as a vital and necessary DISTRACTION in order to apply a technique. On the street, a straight shihonage without atemi will get you killed or at least hurt. Unquestionably.
The Senseis in our dojo emphasize the importance of atemis with two exercises that non- believers in atemi (heathens!) should try, and clearly illustrate a well placed atemi's usefulness:
A- Have a partner perform unbendable arm. Without telling your partner, as you're trying to bend his/her arm, jab your partner in the ribs (or tickle; a kick in the shin also works nicely) with your finger. Remember to keep trying to bend his/her arm as you're jabbing. Your partner will not be able to keep unbendable arm and will accuse you of cheating. But you will have bent his arm- think about what an actual atemi to the ribs (liver) can do. (Enter into technique, such as katadori shihonage)
B- Have your partner throw a punch at you, it can be any kind (yoko, tsuki, shomen). Enter inside of the punch and stick your hand in his face, in front of his eyes, as close as possible, effectively obstructing his field of view or blinding him/her. Do not touch your partner's face (you can also try lightly touching your partner's forehead, or making as if you're sprinkling water in their face, etc., as long as you obstruct their field of view as much as possible, as close as possible). Your partner will instinctively draw back and lose his equilibrium. Think about what an actual atemi to the face (nose, mouth, ears, jab in the eyes, etc.) can do.
(Enter into technique such as tsuki kokyu-ho or sankyo, etc.)
The argument "Morihei didn't use atemi" is clearly untrue: there are many photographs available wherein O'Sensei clearly focuses atemi at an uke to then effectively enter into a technique.
IMHO, training atemis is an important aspect of the martial facet to Aikido. Many Sensei do not obviously demonstrate atemis when they teach a technique; however, that doesn't mean thet they are not used, important or necessary. They are IMPLIED, not IGNORED.