Benjamin has been around the block a few times. He is no fool.
Benjamin was the oldest animal on the farm, and the worst tempered. He seldom talked, and when he did, it was usually to make some cynical remark. (ch 1)
Orwell includes Benjamin for a very specific reason. He is in some ways Orwell's voice.
Benjamin represents the cynical intellectual who refuses to get involved in politics and so fails to affect meaningful change. His cynicism is much like Orwell's own attitude toward life. (enotes character analysis, other characters, Benjamin)
Yet Benjamin has a genuine affection for Boxer. Although he is cynical and attempts to stay uninvolved, he does care. Benjamin is like many people who realize that the propaganda is a lie but decide they are better off not getting involved.
Nevertheless, without openly admitting it, he was devoted to Boxer; the two of them usually spent their Sundays together in the small paddock beyond the orchard, grazing side by side and never speaking. (ch 1)
He supports Boxer, because Boxer is his friend, even though Boxer represents a large contrast. He is the loyal supporter. So it is not that Benjamin is always a vocal opponent of the new regime necessarily. He supports Boxer.
Benjamin does fight alongside Boxer. He is brave. However, he does not go along with the revolutionary rhetoric.
Benjamin was the only animal who did not side with either faction. He refused to believe either that food would become more plentiful or that the windmill would save work. Windmill or no windmill, he said, life would go on as it had always gone on--that is, badly. (ch 5)
In many ways, Benjamin is the voice of reason. But he never actually seems to try to convince anyone to go along with him, and many ignore him because he is always negative.
You can read more about Benjamin here:
The full text and quotes can be found at: http://www.george-orwell.org/Animal_Farm/0.html