Mike, Its probably best to wait until one of the TA's on this formun answer your question, but as I understandit, they get a commission from the cruiseline. You also get the same price as if you called them yourself and booked with their agents.
What is the incentive? Well, many of us old folk are not comfortable using the internet (actually I'm talking with tongue in cheek here as I book through the cruise line, but go with this for a bit) Its just a cost of doing business for the cruise line. Also, if you use a cruise line TA, you are only going to get offered cruises with that line. Many people go to a TA not knowing anything about cruising and need someone to help them choose a cruise line that's right for them and then to arrange everything. That is where the TA comes in. And that is how the cruise lines benefit (by getting new people into cruising)
Why would you NOT use a TA? Can't think of a reason. If you can find a good one, then great. I just happened to find a good CCL cruise specialist and use him to book my cruises. But in RCCL's method, I can't call the same booking agent each time. You call into the main line and are given the next available agent. No personal service. I'd much rather use a TA in that case.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013