I hear this solution so often, it makes me want to gag. Let's think about it for a second: An entire quarter of FotR is about the Fellowship crossing the Misty Mountains, and the Eagles live
on the other side of the Misty Mountains. Let's get past that by having Gwaihir go recruit more of his eagle brothers after helping Gandalf escape from Orthanc. Next, let's assume that the Fellowship wanted to draw attention to where they were going. After all, if the
crebain can track them across desolate wilderness, it would be that much more difficult to find nine giant eagles flying through the air, wouldn't it? Remember that part of their strategy is to not alert Mordor to their plans. Finally, let's just ignore the fact that the Nazgul have the Fell Beasts, which resemble wyverns. Sure, most of the Fellowship had ranged attacks, but so did the Nazgul (one of whom was a legendary archer). The Nazgul had training to ride their mounts, while the Fellowship...hold on for dear life. It may sound reasonable until you start actually taking a look at the details. Sure, the PCs can skip all the encounters except for the BBEG, but is it the DM's fault when they don't have the XP to take out the demigod?