In a discussion about magic systems at Fantasy Faction, I made the argument that the most important thing about writing magic is consistency. Audiences flipp out when characters have a useful ability at one point of a story but for some reason don’t use the ability in other situations when they would be super useful. The most famous example of this is the complaint about The Lord of the Rings, that Gandalf clearly has a way to get the giant eagles to come to his aid and pick up people and fly them around. Just after the Ring is destroyed, the eagles show up and rescue Frodo and Sam from Mount Doom. Right after they completed a foot march of several months! So why didn’t Gandalf call the eagles in the first place to carry him and Frodo to Mount Doom and avoid the whole ordeal?
This is a justified question. But lots of people simply stop there and complain about bad writing. But in fact there are plenty of reasons why they didn’t try that and why it almost certainly would not have worked at all.
The whole point of the Fellowship is to get the Ring to Mount Doom without Sauron even realizing that anyone has any intention of destroying it. But Sauron does not just have scouts and spies to keep informed what his enemies are doing, he also has the power to see almost any place in Middle-Earth. The only way to stay undetected is to look inconspicious and not worthy of being paid any attention. Flying on eagles towards Mordor might be quick, but Sauron should know that the eagles are allied with the elves and the wizards, and there is very little chance they could even get close without being noticed. They are big, they stand out, they are known enemies.
To make things worse, we also know that the wraiths have flying beasts and at least one of them was scouting the area right outside Mordors front gate. The eagles can’t get into Mordor without being noticed by Sauron and once spotted the wraiths are able to fight them in the air. Could the eagles had slipped through or perhaps defeat the wraiths and flying beasts in battle? Possibly. But if they fail Sauron has the Ring and then it’s instant Game Over.
Yes, the eagles are used twice in the story to carry people. The first time they rescue Gandalf from Isengard, but that’s very far away from Mordor and the flying beast could’t get there in time to stop it, even if Sauron noticed it at all. The other time they rescue Frodo and Sam, after Sauron and the wraits are all destroyed. There was nobody there to either spot them or to intercept them, and even if in the worst possible case orcs with ballistas manage to shot them down, it wouldn’t make any difference in the war anymore. They don’t carry the one thing that must never fall into enemy hands at that time. Had for some reason the eagle at Isengard died, it also wouldn’t have made things any worse than they already were at that point. But there are very good reasons why flying Frodo to Mount Doom very likely wouldn’t have worked and why it would have been too risky to try. Two hobbits sneaking in at the back while making a huge distraction at the front door really was a plan with much better chances.