The weapons very well may be attached to his soul but I don’t give a ****. Point is, Black Zetsu said the weapons make him invincible. Itachi can’t maintain Susano’o forever, therefore Itachi isn’t invincible. :Sparks:Ok, and the fact that Itachi had them after being resurrected by Edo Tensei, while Kabuto specifically mentioned that unique weapons could not be created, means that the weapons are attached to Itachi's soul.
So he can never not have them. Maybe he can choose not to use them, but they will never be unavailable to him, and he's been shown to be able to bring out Susano'o in less than 1/1000th of a second, so it's unlikely that he'll be caught off guard without them when they would be needed.