Yavanna's Chosen

[Legate of Amon Lanc]

I figure that Radagast's task was, like that of the other Istari, to help the Free Peoples to fight against Sauron. I think his specific purpose, what he should have done, would have been to observe that animals and such do not fall to Sauron and offer resistence to him, so to say, battle on this field while others are concerned primarily with maintaining Gondor etc., Radagast's task would be to see that not only for example the Men of Gondor stand against Sauron, but also that the animals in the forests of Gondor are not dangerous, i.e. it would be nice to have a land of proud men, but what if Sauron infiltrated the West through the woods, so to say, with some corrupted beasts or whatever. His task could have been also to appeal to the more "hidden" folks of Middle-Earth, like the Woodmen or perhaps Drúedain.

Last but not least, I think Radagast could have been there to give the Istari's mission "ecological" aspect. I.e. prevent that, for example, Saruman would not come and say "Oh, I know how to defeat Sauron! Let's build ten thousand factories and forge unbeatable weapons." Radagast's vocation would have been probably to correct his course at that point.

So, all in all, I think in comparison to others Radagast remained relatively quite close to his task (although we don't really know about Alatar and Pallando), but still, not enough. He was, let's say, lazy, distracted, probably did not fulfil his tasks, did not do what he should. He and Saruman should have had warmer relationship (it is hinted that they were chosen intentionally together), maybe in the optimal case Saruman's eagerness would have kept Radagast active, while Radagast would have helped Saruman to calm down and take into account also others than himself. Alas, neither of that happened.