[akhtene]

Well, it's impossible to argue with the author, but anyway I believe the quotation I gave above can explain the purpose of Radagast being sent to Middle Earth with Manwe's approval. Of course Elves and Men were of the greatest importance, but there were four Istari sent to deal with them, and as animals, birds and plants mattered much for Yavanna and Elves as well, shouldn't there be at least one to care for them and make use of them inthe oncoming struggle.

It's a different matter whether Radagast coped with his mission or failed. But there were birds who could talk ,like the thrash and ravens in the Hobbit and who helped. And who taught them? Anyway, everyone had to play his part, be it great or small.