Ash's face was exceedingly grim.
Right on target.
The Heart Sword is the most direct combat power of the Substitute. The Wind Sword Order disguise as the air to traverse the Holy Domain, but the Emotion Sword is the key for the Substitute to become a Phantom.
Frankly speaking, Ash's talent isn't sufficient to allow him to command the Technique Spirits as seamlessly as he'd like; he simply cannot construct a complete Sword Servant ritual. To put it in Ash's words, it's like having a fantastic idea as an inept boss who just needs a bunch of obedient programmers to execute it.
The significance of the Emotion Sword lies in making the Technique Spirits obedient. Since Ash's talent can't complete the Sword Servant ritual, let the Technique Spirits complete it themselves.
