I don't usually get into these arguments but to discredit Britney's singing abilities just because she doesn't possess the vocal power of Whitney or the range of Mariah would be wrong.
I acknowledge that Britney's not the best vocalist but what's made her so successful is the way she uses her voice to connect to her audience. I've seen many a great singer fall by the wayside because they just belted their way through their careers without any emotion.
"Everytime" is far from my fave Brit song, but the emotion she expresses in that track is all the proof one needs to show that she has talent.
Furthermore, this is just a talent show. Chances are that the winner of X-Factor will probably fade into irrelevance by the time the next season comes around. If nothing else, Britney can at least bring thirteen years of solid experience to the judging panel. If anyone knows about being a superstar, it's her.
And one last point, "Simplicity and style is not grasped without talent". Britney would not have remained the Pop titan she is without being talented. She sings, dances, writes, and produces music and has definitely earned her title of Princess of Pop. Love her, hate her, so what you want about her, she's successful because of her talent and the hard work she's invested in her career.