The benefit of booking thru a TA is their experience and their service--so make sure you choose wisely. You are choosing both the agency and the agent. Then, in addition to great service, some TAs will give you a "thanks for booking with us" gift in the form of an onboard credit, gift card, or other item.
A good TA should be able to get you ALL the benefits you could get booking on your own--onboard credit/discount for on line booking, using a Disney Visa, etc.
OK, so what should you look for in terms of service? Does the TA you are looking at track all their reservations on their computers so that if a price reduction occurs they can nail it for you? Do they track your eligibility for any discounts (FL res, military..sometimes Canadian residents, Georgia residents). Will they arrange air travel outside of
DCL air (or use it if you wish?) Will they recommend or sell insurance (again, giving an option other than DCL's product?) Will they talk to you before, during, and after a booking--on the phone, on line, or whatever your preferred method of communication is? Do they have hours 8-5 or "extended hours?" Do they have someone staffing the office for emergencies? Remember that these emergencies don't occur 8-5 usually!
I've done more than 20 cruises on DCL, and I choose to use an agent. Not because the agent knows the ship better than I do (they probably don't), but they do know hotels, airlines, etc. It doesn't hurt to have someone else on your side when DCL bills you twice for transfers (it was in the package AND billed to my credit card after the fact), when a hurricane hits and you are already at WDW--and you need a room for 2 more nights. Or how about when you get to your connecting airport only to be told that the connecting flight isn't going to leave till the next morning (YIKES). Yes, I've had each of these situations and more. It never hurts to have a professional there to help, even if that help just consists of listening! Most of the time they have solutions/ideas that I wouldn't have thought of.