For one, Pippin always reminded me of Pip the British kid from South Park, I don't know if it was a coincidence or if KC meant for that. But anyways I believe that Scratch didn't send out Pippin on the rat mission at first because like others said, she really didn't have any way to control what he did. If they sent him out he could just run without ever looking back. The reason Scratch sent Kalani was because of his daughter, she knew he would have to obey and come back or else his daughter, which he loved, would be killed.
When they finally did send Pippin to the tower, I believe it was just to find out if Kalani was still alive or if he did die. And the reason they trusted Pippin now was because he had time to feel out the Maulers and they probably offered him some kind of freedom or leadership role if he accomplished this task.
That is pretty much what I believe Pippin's role was. Didn't mind him as a character.