I have heard several individuals make the qualified claim that the explicit use of atemi is not required to train aikido, provided uke respected the implied use of atemi. George Ledyard's article on striking still remains one of the best articles on atemi; in it he not only advocates for its presence in aikido, but outlines the uses of atemi in training. Separating the absence of striking knowledge from the abstinence of striking is important, too.
Full disclosure: I currently train at Ledyard Sensei's dojo.
Ob disclaimer: My opinions are my own, and he may or may not agree with them.