There is a 1/3 chance to get Sage Naruto so if you would have gotten him, it would be a good pull. Since Ao is a common in the kage treasure (lv 70) you pretty much "wasted" the scrolls but you increase the chance to pull sage naruto as next rare.

Like others said, dont buy too many key. What im always doing in this event is saving all my keys until the last day and use them all. If I don't get anything i want, there is always a next time :-)