I decided to use a travel agent because I noticed she was able to get a hold of people and get things done a lot faster than I was when I tried to do it on my own. Also, she was able to provide packages that included flight, airport transportation and hotel stay all in one price for my guests, which seemed much easier than going through the hotel, and also cheaper and more convenient overall.
Because all-inclusive resorts charge per person, getting a "wedding rate" isn't like your typical wedding block reservation of rooms. So you might want to look into that further. I am not sure how it works going through the resort, or if you can even do that without having to reserve all the guest rooms yourself ahead of time, which could cost a lot. Definitely worth investigating.
But my advice would be to go through a TA that will save you a lot of trouble and dealing with your guests' travel plans. A TA will take care of all that, and typically not charge you anything because they get commission from resorts and airlines.
hope that helps!