The nerfed to ninja frags are because this week you only need 25k instead 35k for a rare ninja. It's seem no one really pay attention to that. I think 25k for an event ninja is much better than 35k for a super rare treasure ninja.
Overall the amount of seal scrolls and mood scrolls you gain from it is enough to cover the cost of what you paid. Everything else are extra bonus. What important that players need to keep in mind is the event was mean to gain seal/mood scroll. So if the ninja frags are not someone you are interested doesn't mean it bad, save and wait for another cycle later.

Yea for one or two cycles, after you reach map 10 it's not your main goal anymore but seal and mood are.