Great explanation. We have used a discount, cruise-only TA for all our cruises, and so does our family members and friends. The agent is remote and we communicate primarily by email or by phone. We will not use an agent that is email only, or doesn't provide full service. We usually do most all of the research on our bookings, but it sure helps to have and experienced agent if we do have questions, or if there's that rare problem... and it can and does happen... and not the fault of our agent... the fault has been with the cruise line.
Some people believe that it is quicker, more convenient, and more direct information by booking directly with the cruise line, and are willing to pay the premium to do so for those reasons. This is rarely true. Remember, when you call for assistance at to the cruise line directly, the phone will be answered by a call center employee. Some are pretty good and informed, and others are pretty poor (this isn't much different than about any call center for any company, so isn't a slam at the cruise lines). Most of these people are poorly paid, and don't have that much experience. Many, if not most, have never been on a cruise, and don't really know that much about the ship you may be asking about.
However, most reputable, experienced, cruise-only TA's, whether brick and mortar, or operating over the phone/internet, have been on many cruises. They have the accumulated experience of their own sailings, and the feedback of their many clients. They have enough experience to know that when calling the cruise line, if they get a representative that is less than competent, to hang up and call back, or kick it up the chain of command until they get someone more capable. Many of us wouldn't know that kind of stuff, so you'll hear lots of contradictory reports of information directly from the cruise line.
We use one of the TA's that advertise on this board, and is often mentioned and recommended. Our TA has been of invaluble help to us on several occasions. Her prices are low, if not the lowest... however we will keep going back to her not just for the good prices, but for the great service when we need it.
paul