This is definitely not a new idea for Zelda games, the rod of seasons also carried the season spirits. Each season spirit you free the rod gains the power of that season. They appear as ordinary fairies in the game.
The phantom sword draws it's power straight from the Ocean King in the form of force gems made into the sands of hours powering the sword.
"Hmmm. I cannot call it truly complete without a handle. Take this blade to the Ocean King. He will add the Sand of Hours. Only he can empower the Phantom Sword with the ability to control time." — Zauz
"The sand inside the Phantom Hourglass is called the Sand of Hours. The beast you defeated in the Temple of Fire held some grains of it. It is said to be made of Force Gems of the great spirit, the Ocean King. It gives deep strength to whoever holds its power. From now on, you must defeat monsters and collect their Sand of Hours. For the more sand you place in the hourglass, the longer its power will last." — Oshus
The Four Sword also gains power from force gems
"Force Gems can be found all over Hyrule. These gems are the source of the Four Sword's power.
Which could be related to the force fairies that are released from the blade when Link dies.
Even the master sword of TP was powered up by the Sols who willingly gave their power to the blade.
"Amazing... The power of the Sols has been transferred to your sword... The guardian deities of my world are on our side, too! You really are the chosen one, Link... A true hero! As long as you have that sword, you should be able to repel the shadows Zant has spread!" — Midna
The Light Arrows are given to Zelda from the Light Spirits in TP. They do not reside within the arrows but are more likely a part of them shaped into an arrow.
"Spirits of the light! Wielders of the great power that shines far and wide upon the lands of our world... In my hour of need, grant me the light to banish evil!" — Princess Zelda
Not only weapons but other items hold spirits. The Goron Mask of Majoras Mask held the spirit of Darmani who died on snowhead. And the Zora mask held the spirit of Mikau. The Fierce Deity mask is also alluded to have contained a spirit within it.
It appears to me that SS might expand on the concept that certain objects have spirits within them, while some might gain their power directly from those spirits. The concept of important objects housing powerful spirits is not new in Japan either. They were called Tsukumogam.
Tsukumogami (付喪神
http://en.wikipedia.org/wiki/Help:Installing_Japanese_character_sets, "artifact spirit") are a type of Japanese spirit. According to the
Tsukumogami-emaki, tsukumogami originate from items or artifacts that have reached their 100th birthday and thus become alive and aware. Any object of this age, from swords to toys, can become a tsukumogami. Tsukumogami are considered spirits and supernatural beings, as opposed to enchanted items.
http://en.wikipedia.org/wiki/Tsukumogami
