I would say, taking the explanation given, that it is one of the most fundamental principles in Aikido from beginning to end. As Mark says, it's a 'sliding scale' or to me a rising scale of ability with regards to that one thing.
I fail to see how you could do good Kaeshi Waza for example without it.
In life this same principle applies in so many things, listening being one. Some are good at it some are not. Some need more practice.
There is seeing and there is looking, there is copying and there is duplicating, there is hearing and there is listening, there is collusion and there is being with. In each case one is following as described and one is not.
There is Ki out and there is Ki in.