You are making two different and opposite arguments.
I agree that the only reason to book a cruise either with Viking and then transferring to a TA is the OBC. And please note that my TA gives it to me as a refundable credit. I have received a check for the unused credit from Viking many times.
I don't agree with your last statement. I expect (sometimes foolishly) that Viking knows more about their product than a third party who sells different cruise lines. If I call my TA and ask some question, she will likely have to call Viking. That doesn't make things easier.
In fact, after you book a cruise thru Viking and transfer the booking to a TA, the TA then must call Viking for the sole purpose of transferring a booking. That does not make Vikings time for efficient.