Duncan Garner has clarified why David Parker didn’t replace Cunliffe on The Nation in blog a comment:

duncan garner May 15th, 2012 at 08:54

Morning,

The invite wasn’t for David Parker – it was for Cunliffe. Parker is coming on next week’s 3 News Budget Special and also on The Nation’s Budget Special two days later. That’s why we invited Cunliffe on. Parker is already booked to come in. Twice. In the end Tony Ryall was on our show and Peters and Norman on Q n A. Where was the Labour representative? They need to do better than this. In my view Cunliffe wanted to come on. He was gagged.

Cheers
Duncan

That clarifies it quite a bit.

But the answer to “Where was the Labour representative?” was still a dual responsibility, Labour chose not to put Cunliffe there, and TV3 chose not to put anyone else from Labour there.