Jean Grey's abilities have always been a plot device, and are as strong/weak as needed to make a particular scene happen. She's like Dr. Strange in that respect, where if they make a film version his powers will not be identical to the comic book versions, where his power fluctuates depending on plot needs. If Jean Grey was as powerful as she is sometimes depicted in the comic books, in the movies? She could have stopped Magneto and all of his mutants by herself without breaking a sweat. But that makes for a boring film. I get that they were showing the limitations of the character, and wanted to blast off a Phoenix nutshot. But it really was just another Hollywood reason why a scene had to happen.