I think that at the end of the Kabutomaru fight, Itachi gave the legendary weapons to Sasuke. Why? Because Sasuke Itachi knew he wasn't going to use them and he still had hopes that he would turn good, so he gave them to Sasuke in hopes that he would be able to use them for the benefit of good like e.g sealing Madara or the Juubi.
Also, having the two will make Sasuke stronger as the shield can reflect ANY attack. Naruto's Bijuu Bombs would annihilate Susano'o otherwise. The Sword could possible limit Naruto in the stage because a cut from it would take chakra away, dwindling Naruto's time in the form.
Tell me what you guys think.