That kick back is already worked into the quote from the bank. Like casinos the house always wins.Well in our case it was mostly because of the sheer convenience.
They apply to all 4 banks at the same time and send you each quote as received. Then you ask them to renegotiate and play the banks off against each other.
No clue about the negotiating privately. Probably true though. But they already have much better relationships with the banks so I let them do the negotiating for me. And they do try a bit harder when you ask for better deals. Because in the end they get their kick back from the back. Doesn't cost you a cent.