The advantage to using a travel agent is that you get access to ship information from a number of cruise lines - not just one.
I guess in this day and age a lot of people feel confident enough about picking a ship that they bo longer need a travel agent.
Personally - I will keep using one.
Look at the new rules for late final payments from Princess and Cunard. If you are a day late your cruise is cancelled and you lose your deposit - no warnings, no grace period.
A travel agent would (should) not ever let than happen to you. She would call you a week ahead of time and remind you of what you need to do.