User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: SECOND MATCH!/@comment-4829046-20130724065721/@comment-7418318-20130725074405

All the more reason to show Sasuke try and fail. There is a chance it might not work isn't exactly a good reason for withholding an attack that won't backfire in any way in a battle.

It's not like he would lose a lot of chakra, start bleeding from his eyes (now that he has Itachi's eyes) or lose the moment of surprise.

And while it didn't manage to annihilate the Juubi completely, it still hurt it. Right now hurting Obito enough might cause the Juubi to go out of controll again and leave Obito to self-destruct (something I really hope will happen).