If Demonic Statue of the Outer Path requires only a part of the chakra of the Nine-tails and the Eight tails, why did they have to kill the other Jinchuuriki to obtain the tailed beast just to complete the Juubi?
Why not obtain only parts of the chakra of each tailed beasts?
Having the chakra of kyuubi and hachibi will suffice, but the Juubi is considerably weaker. Having the full kyuubi and hachibi will essentially create the juubi in its perfect form. For each bijuu that juubi doesn't have, the monster gets weakened.
It's basically like comparing a fraction of the kyuubi's chakra vs an entire kyuubi with his full chakra at your service. The amount of power is significant.