We must emphasize a balanced training of mind, body and ki. Always remember that the right attitude towards life leads to efficiency and harmony. One-sided training will create a lopsided person.
Training the mind alone disassociates the mind from the body; the mind will be calm, but the body will be empty. The techniques might look smooth and beautiful, but will be ineffective and unrealistic for self-defense.
Over-emphasis on physical training may also lead to embarrassing and dangerous situations. We have physical limits and there will always be someone stronger or faster. Relying on physical strength can lead to dependence on brute force. Using excessive force to control others could injure them when successful and yourself when unsuccessful. Finally, there is always a tendency for collision when physical strength is not controlled.
Developing your ki or "inner force" is very important. However, we must be very careful about the proper use of ki. Merely training to develop strong ki can lead to haughtiness, and eventually, that person will encounter someone who controls his/her ki more efficiently. Over-exerting your ki can be dangerous to your health. Ki is your life force, and you need it to sustain your life. Needlessly extending your ki can lower your immunity against sickness. With proper training, one learns that very little ki is needed to harmonize with and control an attacker. O-Sensei emphasized ki no myoyo o tadashiku "the proper usage of the mysterious power of ki".
Use your ki in moderation and use it to bind your mind and body together for a harmonious way of life. Training the mind, body and ki simultaneously to harmonize with the ki of the universe is the true path of aiki.