To continue on from what Angry Brandybuck just said, dropping Frodo off near Mt. Doom would require the eagle to land and, not being the smallest of creatures, they would probably have a hoarde of orcs waiting below them, shooting arrows etc.
On the fell beast/eagle subject, then if the nazgul had some long range weapons then I doubt the eagles would have too much of a chance, a couple of firey arrows could probably bring an eagle down, it might not be killed, but a bird can't fly without its feathers.
The eagles were just completely against the whole secrecy tactic, not to meantion you'd have to do a whole lot more than just heal an arrow wound to persuade them to agree.
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"We might succeed in roasting Pippin alive inside." - Frodo.
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