What we found is that the actual cruise prices are the same, even if you book direct (which we did our first cruise) but where the perks come from a TA is all the perks they give you on top of the Cruise line deals.
Last time we got Prepaid Tips and free Specialty Dining for two, On Board Credit and they kept on top of RCI rates and got them to match a price drop for us, in addition to the RCI OBC.
This time so far we got Loyalty On Board Credit plus 10% of cost of our cruise On Board Credit in addition to a nice RCI OBC. Their OBC will cover our Tips and Specialty Dining.