Until he learns that as a number 8 he has a responsibility to actually take his batting seriously depending on the situation (and one of those situations is when chasing a follow on total and batting with an actual batsman) he should be at number 11. Just a horrible attitude with the bat. And this is a guy that is touted as a potential vice captain. I sure as hell hope not.
Think he's been given licence to hit out, witness his partnership with McCullum. Don't know if he's able to defend.
Also as someone just pointed out, it might be his place under threat from Bracewell! Though I'm still unconvinced by Wagner's ability to threaten consistently and in all conditions.
Thats my point though. Assuming you are talking about his 1st test partnership, that is the exact situation he should be given a licence to play like that. When you are 6 down, 50 behind the follow on, and batting with a batsman is not the time.