I don't generally use a travel agent for Caribbean trips. If you're simply booking a resort in a mainstream place on which there is lots of information available, and a simple air itinerary, it might be less trouble and same price to book it yourself. If you're going someplace off the beaten track, or need to deal with some of the regional airlines that may have issues like lousy web presence, poor or no online booking, unreliable schedule info, whatever, then there can be a real advantage to an agent that keeps on top of this stuff.
Agents are also handy if you're short on time, unfamiliar with travel planning, unfamilar with a region, or just don't like doing it. BUT - still educate yourself as much as possible on where you are going, places to stay, and how to get there, as it's the only way to make sure an agent is planning the trip that suits you best.
For the Pacific region, I've come to the conclusion using an agent more often makes a lot of sense.