So in the end, as a former fan of the series, this movie only works on yes men. I don't follow like that. I said it before, if it's crap, I will acknowledge it, even from my favorite. And this is crap.
My theory is that they tried to appeal to girls as well and insert Hinata as someone whom Japanese girls could identify with. She's pretty in a very traditional Japanese way, and her manners are so meek that she couldn't offend anyone.
Sakura doesn't look like anyone except herself, and her character has no problem making ripples. Only some girls would identify with her, and only some guys would be into her.
I think they went with Hinata to seize a bigger chunk of the audience, sell more movie tickets, more cds and more merchandise.
Who's going to see a ninja-romance movie? Girls.
Who's going to by a cd of love songs? Girls.
Who's going to buy a calendar or an alarm clock or a keychain? Girls.
Hinata has actually replaced Naruto in terms of the marketing importance in this movie. It's about Hinata, and the sales are geared to girls. It's bet they're banking on it being a date-night movie.
I think it has nothing to do with what's come before in the manga. Only what they've decided will sell the best going forward.