Who was the second best player on the Mavs championship squad?
You could have a case that Terry was the second or third best player on that Mavs team if you were talking about the playoffs only. Otherwise, Nowitzki, Chandler, Kidd, and Marion were all more important.
The thing that was cool about the 2011 Mavs was that other than Dirk Nowitzki, it's hard to say that any of the next four guys on that team were any more or less important than the next guy.
They all had different roles that they played, and they all played them well enough to get that team a title. Terry's role was primarily as a secondary scorer behind Dirk Nowitzki. He played that role well in the regular season, and then stepped it up another notch in the playoffs.
He also showed that he was a serviceable ball handler and distributor and a gritty competitor on the defensive end.
I don't think that you can make the claim that Kidd, Marion, and Chandler were undisputedly more important to that team's success than Jason Terry during any part of that season, regular or post.