Moving on: As for the effectiveness of atemi, I find that rhythm has a lot to do with it. The element of surprise, of striking in a "syncopated" way, i.e. right before it is expected, and to do it paying attention to the movement of the opponent (either with it or right against it), and so on.
Atemiwaza is a martial art of its own, if studied profoundly. And true: when you know it well, you don't need any aikido techniques to follow up - but then, of course, it is no longer aikido.
Well, I'm not a big fan of Nishio's atemi style for the reason that it seems to be done too much as an external art applied in Aikido. In that way, I agree that if one only did atemi then, it might not be Aikido, as you said.
However, to me, I think that there is what can be called "Aikido atemi". As someone mentioned, you could say that a strike is a strike, no matter what you call it, but the reason I call it Aikido atemi is because the application and method of striking in Aikido to me seems different from other arts
Here is a clip of one of my favorite Aikido gurus doing atemi. He applies atemi many times without doing anything else, but you can still see the technique. Even though he does not do an irimi nage, and only does atemi, it is still there. In this way, the atemi is uniquely Aikido because the principle and essence of technique, such as irimi nage, is necessary to properly execute the atemi.