I like atemi; to me it's kind of like kiai-it softens up the really hard cases for my waza.
I'm not sure why O-sensei did it so much; from everything I've read and heard he was temendously powerful and well-balanced even in his eighties. It doesn't seem like he' need atemi to complete his technique. I do because I'm kind of a wimp, in all honesty.
Did O-sensei ever explain why he used atemi?