[amyrlis]

I think it's more because the Eagles did not typically "bear" people on their backs. It was a great favor that the Lord of the Eagles did for Gandalf in rescuing him from Orthanc and bearing him to Rohan. He did it because he is in debted to Gandalf - Gandalf saved his life once by healing him of an arrow wound. It would have been too much to ask for the Eagle to carry Frodo all the way to Mordor. Also, the Eagles are "servants" of Manwe, kind of like Gandalf serves the Valar. Gandalf can not use his power to rule men, only to guide them. I think the Eagles role is much the same - they can provide aid, but need to remain a bit removed from the troubles of Middle Earth. Anyway, just my thoughts.