By doing so they would begin to flow and become more effective.
In the sense of Aikido thats true, but not all systems are based on the concept of flowing. Many of the other arts do not stress flow and there in lies the difference. Personally, I don't think the study of principle and concepts is done alot. For the most part, people are technique oriented. They tend to deal in absolutes and stay with in the box. There are other arts that teach root techniques or mother techniques yet the student gets bogged down in the technique and never expands the knowledge of the principles or concepts that are involved.
On the ohter hand many Karate styles are based on strong, powerful linear movements and people never get to the level of seeing the flow in technique. Some styles don't employ it. Others focus on breaking bones, and destroying the attacker. While principle and concepts are involved, very few employ it. I have seen this in my Lua practice. Most Hwaiians use the principle and concepts of the art and the tenents of their culture within the art, and are not technique oriented. However most in the states are technique oriented. They want the techniques now, and do not learn 'about' the techniques.
So here is where the hard and soft comes in. Its the appearance of what they do, but more so, its a mind set and an approach to the art. In Aikido, you have flow, and key extension and basically go with one's joint. Another Jujutsu style is right there, no extension in fact bring them in(take out the space or voiid)and go against the joint, which once again is a harder approach. There are others that are 'break oriented and all locks are breaks, so compared with an Aiki appraoach they are harder.
It a subjective thing, and its a comparison between the arts, which I feel is wrong to begin with. Its funny as Goju Ryu Karate(which means hard/soft)employs soft and hard movements,(in this case hard and soft is defined by the differences like blocking and parying)yet no one ever questions why there is hard and soft movements. Circular and linear as well. It seems that the more linear, the harder you are viewed, and the more circular the softer. Its an appearance and a mis-conception, but that is how its viewed.
I agree the discussion of principle and concept is more important,but its not one alot of people deal in. WIth more and more people training, and more interpretations to what is being done, styles and styles of styles are begininig to define the arts and their approach. People will train in a style then change 2 techniques and call it their own. About 10 years ago I trained with a Aikidoka Sensei who took kotegaeshi with one hand and the other, curled the fingers into the hand. He did ikkyo and added a sweep to the takedown. I asked him why and he said to make his style of Aikido harder. He also employed alot of atemi waza, again to make his Aikido style harder. So this is what you deal with a perception of what makes things hard and soft.