Bryan Singer was approached to direct X-Men in the mid 90s and turned down the offer several times because he didn't take comic books seriously. However when someone sat him down and had a deep talk with him out the deeper message of X-Men about prejudice and discrimination, he had a change of heart as the story stuck a cord with him as he is openly homosexual. The difference between Singer and Ratner, is that his heart is truly in the message of equality and not just mindless action. That is why X-Men and X2 were so great, because they had so much substance that the action didn't matter. See Superman Returns didn't work because he didn't get the character, X-Men 3 didn't work because Ratner didn't get the characters either, no one can understand the X-Men better than someone like Singer IMO.
The screen writer really doesn't have that much power, the medium of film deeply engages the senses, and as such the style the director gives the material, along with the score, and the actors portrayals ultimately has a much greater impact than the script itself. Look at The Amazing Spider-Man, the script is basically a scene by scene revision of the 2002 film, really if you look closely besides the origin, the same stuff happens at the same time (Peter gets bitten while having awkward banter with the love interest, uncle ben dies after lecturing Peter about flash, Peter says "You're not my dad", The villain has a crazed talk with his two personalities after discovering Peters identity, "I can't be with you") but it was original and great because of the vibe Webb and Garfield gave it. I'm sure Singer isn't going to disappoint regardless of the screen writer.