1) We have not in fact had "great" X-Men scores before XMFC. The closest we came to a GOOD X-Men score was X3, and really that's just because 2 tracks stood out (Dark Phoenix Theme and the Main Title). Calling X1 or X2 "great" or even "good" soundtracks makes my musical heart die a little inside.
2) I stopped being "excited" about this film when it was announced they were doing DOFP. Then I switched to "less than thrilled" and when Vaughn left I switched to "very apprehensive" and then at the announcement that Wolverine would be back for more than a cameo I moved on to "desperate for some good news, ANY good news". At this point the ONLY thing that can save this movie for me is the composer. Fassbender and McAvoy will be undoubtedly brilliant again, of course, but great acting alone cannot save a movie (see: Haywire, Penelope).
3) Don't be so easily dismissive of composers. They are just as critical as directors when setting the tone of a scene, even if people don't take note. Sometimes composers can save a movie all on their own. The new Tron movie was a piece of garbage, but because the soundtrack was bloody fantastic, it made the movie worth sitting through. Assassination of Jesse James? Pretty take-it-or-leave it movie, but the soundtrack is brilliant. And can anyone imagine Amelie without its telltale accordion and xylophone tracks?
Similarly, many an interesting movie has been made unwatchable by poor musical decisions. The score is one of the main reasons I have a hard time watching X2 now. A composer, and a director, need to know what music is appropriate and when. How to create a memorable theme. When to push forward and pull back. It is NOT a good sign that I need to refer to Youtube every time someone mentions John Ottman. At best his stuff is forgettable. At worst it has the power to run this movie into the ground all on its own. To me, announcing his return would be the equivalent of Singer stepping down and handing the reins of DOFP to Brett Ratner (though I'm sure Ottman is a much, much nicer person). Yes, the music is that important. Especially when you've developed such a concrete, character-associative theme as Magneto's.
3.5) It's reeeeally not up to you to determine whether or not my reasons for sticking around are "good enough", or to assign value (or lack thereof) to things that are important to me about this movie.