If you name your TA while booking onboard, it is not considered by
DCL to be a transfer. Our TA gives us $100 for short cruises, $200 for 7 nights or longer. This is in addition to the OBC from DCL for booking onboard.
If we transfer to her, the OBCs are half. The reason is that DCL gives TAs a reduced commission for transferred reservations. They feel that the agent who actually booked the cruise has likely done much of the work. The OBC she gives us is basically sharing a part of her commission with us as thank you for booking. As a pp said, the TA does the grunt work. I like not having to sit on hold with DCL and she does whatever I ask her to do. To me, it's a win-win situation.
I miss those days when the OBC for booking onboard was tiered, with platinum members getting $350/cabin for booking onboard instead of $200. All those OBCs really added up. For Alaska we splurged for 2 cabins (2 teens) so we got $1100 in OBCs that cruise.
I love OBCs.