Because they are still going to have to pick and choose which ones and your Ignoring the fact that Bryan Singer has never been one to just pack up screen like Brett Ratner and the origins fiasco.And It doesn't take a genus to figure out he would be more Intrested In getting back the actors he cast than the actors Brett Ratner had. If you think he or Matthew Vaughn want to call attention to anything that went down In The Last Stand your very mistaken.Hell The wolverine may not even have direct reference to events of Last Stand beyond Jean's death. Rogue Is almost certinly going to fill the role Kitty had.She may not be time traveler but good chance of her filling role In future scenes.It doesn't take a genus to figure out the stron possibilty of him wanting to spotlight Anna Paquin.Ellen Page may not even want to play Kitty again.She had to be begged to audition for the Last Stand. Kelsey Grammer Is on record of saying he hoped the Last Stand would be the last X-men film.Witht he different looling beasts In Last Stand and First Class do you really think they will want to call attention to that. Shawn Ashmore and Daniel Cudmore will be In film before any of Last Stand actors. Singer never said Days of future Past would celebrate all films.He talked about connecting dots between films. If the fuutre Is presented as totally alternate that's even less reason to use any Last Stand actors.And If halle Berry,Anna Paquin,Rebecca romijn,james Marsden,and Famke Janssen are all joining Patrick Stewert,Ian Mckellen,and Hugh Jackman In future scenes that Is pretty large group of characters to service In future scenes.