Just a comment on the "reward not so great" part....

If you ever looked at your server list and noticed some guys are super far ahead in experience over everyone else you c*ually blame Nine Tails for that. Jonin medal and stamina purchases don't give much experience. But a Nine-Tails even can give over 300,000 experience. That's more than what you get for most Main Story missions and it's usually about 20% of a level. If you get high amounts like that ALL the time because you pay to win then you will level much faster than other people.

So if you want to keep up with power and levels the Nine Tails even is crucial.