I don't think relying on your tools is inherently a weakness. After all, what would a puppet ninja be without his puppet.
I think using tools that have their own personalities and can choose not to follow you...probably a mistake.
Not that they've said.
In fact I think they said it was the Sword of Kusanagi. Which is Orochimaru's sword, even though Sasuke's looks nothing like Orochimaru's.