Robe colours

[Legolas]

I'm fully aware Radagast didn't fulfill the mission of the Istari. I never said he did. I just told his real motivation for going.

...it's never really explained why she wanted him to go and he doesn't seem to have served any real purpose except to alert the eagles to watch out for Gandalf.

You seem to miss the point. You said you didn't see any reason for Radagast going, and I cleared that up. Yavanna wanted him sent so she could have someone watching over nature up-close.

Manwe could've easily guessed that Radagast wouldn't stick to the mission. However, his proposition was to send three Istari, and the two that volunteered, Saruman and Alatar, along with Gandalf, whom Manwe hand-picked, were enough for him apparently.

Yavanna started whining. I'm sure his response was "Yeah, eh, whatever, tree-hugger."

There was a reason for Radagast going, it just wasn't the intentions of the Council of the Valar. He failed, as far as the "real" mission was concerned.