Not to beat a dead horse here... but I have done it both ways. Through direct booking with the cruise line and through a TA. I prefer the TA because I don't have to worry about it. IF you get a good "personal vacation planner" with Carnival it can be a pleasant experience. But they are not going to watch for price drops for you (why would they) and they will only book what you ask them to instead of say... finding a balcony and a inside room close by for nearly the same cost so that you are not crowding a family into one cabin... If you get an inexperience PVP or one that quits before your cruise... well good luck with that.
On RCCL you don't get to have a PVP... you get the next available agent each time you call which can be a BIG hassle.
If you have a TA that you have dealt with and like them, there is no reason not to continue that. BUT if you are talking about a discount travel agent online then the same issues can arise as having a PVP.
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