No, Arby was interesting in Halo 2 never taken to his full potential but still interesting, he provided us insight in the Covenant.
Now you cannot defend Arbiter in Halo 3, he has no point in begin there are said before, he has so few lines, fewer than Master chief himself!
What meaningful thing does he do apart from shake Lord hoods hand? What does he contribute to the story?
Nothing, he was a wasted hollow character in Halo 3 and just seemed to be there because, he was in Halo 2 with no real reason why he was there.