I think that due to the themes of what X-Men is all about, these movies NEEDED to be grounded.
The reason why I love X-Men so much, as both comics and movies, is because they are a balance between fun (mutants with amazing abilities, battlings fantastic villians with extreme plans for global domination, or what have you) and heart, character, and emotion (struggling for acceptance, fighting against oppression, and the family element these characters have among each other)
Because of that, X-Men becomes more than just some typical comic book about a superhero fighting super powered villians who become super criminals. I don't think that characters like Spiderman, Hulk, Fantastic 4, Ghost Rider, etc... have even a fraction of the heart and depth that X-Men has, and that's why I like X-Men so much over those other characters.
And that is why I love the way the X-Men movies were made, because they actually focus on what makes X-Men different from other comics, while maintaining the fun and entertainment of being a comic book.
To me, I don't mind that there is no Phoenix raptor, I don't mind the black costumes, and I don't mind the toned down powers, because to me, X-Men isn't about power displays and bright costumes. It's about a group of outcasts who risk themselves for social equality. I don't care that there was no M'Krann crystal, or distant galaxies, or anything else like that, because the Phoenix struggle isn't about all that, it's about one woman's descent into darkness, and what her loved ones would do to save her. And yes, while I wish things were handled differently, the Phoenix Saga in the movie maintained enough of it's essence for me to be able to kind of accept (i.e. Jean Grey still wants to die because of what she has done to her loved ones, which, even if not as blatant as it should have been, puts her love with Cyclops to the forefront of her plight)
In the end, the way the X-Men movies were handled, in terms of the balance between realism and fantasy, was handled pretty much perfectly. As X-Men: The Last Stand showed us, I believe you can throw in a bit more fantasy, and still maintain the realistic setting, and maybe Singer's films could have used a BIT more comic book goodness, but in the end, I'm still glad that the movies took the route that they took.