The biggest advantage to a TA is they can do things other than Carnival. I can book a Disney cruise, a trip to Hong Kong, or whatever I want with my TA.
A PVP will not advocate for you when your interests are opposite Carnival's as you are experiencing here because they will not bite the hand that feeds them (understandably so). The only advantage to a PVP is you have a consistent voice on the other end of the phone when dealing with Carnival.