I wonder....perhaps Jackson chooses to portray Radagast in this way as some means of making up for the loss of Tom Bombadil. That essential part of Middle-earth, the spirit of the wildwoods and of the land itself, has been completely missing so far. The Elves can't provide that element as they are not 'of' Middle-earth, and the Ents can't provide it, certainly not in Jackson's creation as he chose to make them more like trees and less like giants. To have any 'picture' of Middle-earth without the wild spirit is to have a picture that's not complete.
And, had Jackson not been so stupid as to excise Tom Bombadil from the original films, then the Radagast we get in The Hobbit might not have been so jarring to some viewers.
Now for a detour into some wild territory...I was reading a novel last night where a character makes use of Tarot cards and it got me thinking how Radagast reminds me of The Fool (and Tom Bombadil does, too). He is a loner, wild and free, existing in an almost liminal state and literally mindless of any of the restrictions that society places on appearance and behaviour.
The wikipedia page is as good as any if you are not familiar with the Major Arcana and makes a note that in very early decks, The Fool was often portrayed like a tramp with raggedy clothes and feathers in his hair. Very much like Radagast in the film.
Which goes back to what Boro says about Jackson making use of archetypes. Actually, given that Radagast is such a slippery character for even very keen readers to get a handle on, is it any wonder Jackson has gone back to an archetype?