wanna know the REAL reason Hashirama is back??
it's cause Kishi had no idea this Anime would be this successful and still be running after all these years. i HIGHLY doubt he had a perfect 10+ year storyline all set and ready to go.
when he decided to make Orochimaru revive the 1st and 2nd Hokage a long time ago when he fought the 3rd Hokage, it was an awesome idea back then, but now, at this point in the storyline, i'm sure he was like, "crap... how can i make this work??"
so, with this Anime series still going, he has to fill in the holes in the storyline somehow. i think he's done an awesome job.
when you start viewing things from a writer's point of view and not a fan boy's, you'll start to see how it makes sense. =)