Don't agree. Saito clearly shows that he can fight the familiar spell up to a point and he sure isn't like a robot. Robots are incapable of bitching and complaining about something they don't want to do even if in the end they are forced into doing it.
And that's the growth he goes through. At the beginning he fights his role in pretty much every way possible. But he eventually warms up to his role and WILLINGLY accepts his tasks. He doesn't act like a slave and he isn't treated like a slave.
In fact the way he acts is *gasp* quite logical. Its like being tied down to a job you hate. You bitch, you moan but you still go every day because you have to. But then one day you meet a girl at work and then getting up and going doesn't seem so bad. That's a much closer analogy to the situation Saito is shown in than claiming he's a slave or a robot.