What do you mean, "need"? Does Toei not think a movie can't stand on its own merits anymore? I've noticed this trend of having a merchandising gimmick for every Sentai and Rider movie started this decade (Imagine that!). Of course there have been great movie-exclusive Riders, but where did they go? They died out with Faiz, and briefly came back in Hibiki and maybe Den-O.
Now granted, I actually liked Flight Mode in concept. But what ruined it was that nothing about Emperor Form lived up to the "Kiva is a dangerous monster than must be suppressed" idea that was hinted at in the beginning. It was just a giant bird-like dragon that appeared thrice.