I agree with Kelly, you adjust to your partner. If I train with one of our more experienced students and he tries something (a stike, turn, movement that is not incorparated in the technique we are training to see if it can be done otherwise) I react instinctively. Meaning I strike back or counter my movement, this useally results in me hitting him, let's see, elbow on nose, stike with jo in his neck, push him in one movement flat on his back to the ground and this is just the past 2 months,
. No injuries resulted from this all, I have a good control of the force with which I apply and he knows that if he tries something he gets a reaction from me. I still keep apologising and he still keeps complimenting me on a good technique / countermeasure after I hit him, very dual but we both learn something.
However, I would never do this with a beginner, they often do not expect an atemi or anything that differs from the movement they have just been shown by our teacher. Furthermore, they often lack the reflexes which after some years of training prevent the uke from getting hurt when trying something different / unexpected.
Aikido is not about hitting people but to unbalance the centre. You adjust to the person you train with, an experienced aikidoka is a lot more difficult to unbalance than a beginner. So what you see more experienced aikidokas do to each other is real, at least in our dojo, but training with less experienced aikidokas they adjust and subsequently the real strike becomes something else, a push or a touch or sometimes just a word.