I use a TA all the time, I have NEVER paid "sticker price" for a cruise, the TA always has group space and the fares are always cheaper (sometimes considerably) the deposits are fully refundable, and there is always some type of perk thrown in (OBC, dining, wine.....) Also, some of those premium spots on the ship that show sold out when you look on the Royal website, are actually available and being held by travel agents for their customers.
The most important benefit, to me, without question, is anytime I have an issue I email the TA, and it is handled in a matter of hours. No phone calls, no dealing with offshore customer service, no phone trees, and no endless waits on hold.