I've only cruised twice, both times with
DCL, both times new itineraries (Alaska 2011, Iceland 2015), and both times I booked myself by calling Disney at 8AM on opening day (for general public and silver
Castaway Club members). Even at those busy times, the CMs were great and very helpful.
I tend to research and book all our vacations and flights though. I like that kind of stuff.
I very much like to have control of my reservation ... if you go through a
travel agent, they "own" your reservation until you board the ship basically. DCL will not deal directly with you regarding any changes to your reservation ... it has to go through the TA.
That said, after two cruises, the idea of OBC is appealing. I might consider using a TA if the OBC (or equivalent) was pretty significant. Don't think I would do it for $25 per stateroom though, but that's just me.