Yeah, Ellen Page returning was a great, pleasant surprise. To her as well! She has said a few times she never expected to play Kitty again. For a long time it never seemed the story would ever continue after The Last Stand.
Clearly Singer thought she was very well cast. Even though he had nothing to do with it. I suspect that Kitty's role is still very important in this, even if different from the comic. If it was just a small cameo like some of the others likely are, I doubt she would have done it. For Paquin and the others its a family Reunion with Bryan Singer.
Page returning creates a strong consistency of the team members between the films. After characters like Nightcrawler and Angel only appeared in a single film and really were not officially X-men. Plus Kitty had small cameos played by other actresses in both the 1st and 2nd movie.