Perhaps I am missing the point but it seems to be the ability to adjust to the current situation and apply the correct amount of hardness, softness or combination of the two allows for the greatest amount of flexibility, options and opportunity for success.
Are you talking about the training or about the application of internal power? When talking about application I can agree with you. The wider your tactical options, the better. With regards to training the question boils down to how much do you want to rely on muscle? So you have to make a choice there. Even if it would be possible (which I don't think it is) to only practice exercises that are on the limits of the hardness-softness scale, you'd still average out somewhere in the middle.