I don't think leading the Covenant Separatists, and killing the source of all his problems is nothing. It just seemed that way because he didn't have much to say. He knew his goal and what he had to do and got the job done, without much chatter, at least until near the end. I wouldn't say that he did almost nothing when humanity would have been vanquished without him and what he did.
With that said, he was in the background too much in Halo 3, and he needs to be less of support and more of his own guy again like in Halo 2. I also hope that since you will probably not be playing as Arbiter, that if you do Co-op the second character acts as him.