I was just on another website that I frequent (Kotaku), and the discussion of, "Why didn't an eagle carry Frodo to Mount Doom?" naturally came up in the comments section of the review of War in the North. So anyways, my point is; how can people still overlook the fact that Sauron had thousands of ravens under his control? The common argument is: Nazgul would stop the eagle. The common counter argument is: The Nine didn't have their Nazgul right away. They were on horse back for a big part of the time. My counter argument to both is: Thousands of ravens. Am I crazy?
The simplest argument, to me, is that the ring had to be taken there unseen. If they had tried to fly it there, Sauron would have sensed it and also easily located it, at which point he would have been in a position to do something about it (or at least to attempt to). As it happened in the stories, I'm not certain Sauron even realized the intent was to destroy the ring until near the end. Flying it straight into Mordor would have been a risk not worth taking. Whether it was ravens, nazgul, or something else, keeping the location of the ring and the intent of the Council secret for as long as possible seems paramount.
I always thought that if they tried that they would be spotted too, and if not killed by Nazgul or the crows then he'd probably just get shot down. After all, they would need to fly low enough to drop it into the volcano safely...
I just realized I said "The nine didn't have their Nazgul right away." I totally meant to say fell beasts. Haha whoops.