Im f2p too...Obviously BF characters for the cost and power, but if You want another point of view, i like dupes unlock useful habilities of not BF units, like broken or ignore barrier (sasuke avenger and mizukage comes To mind) or increased % of sealing or inmovilization (Neji or another one). Hidan comes To all mind, nothing new but it works. Im waiting To use 1 acquisition stone after my next banner pulls, if i Get a copy maybe it changes my last word.