i usually do work with the wedding coordinators at the resorts and usually can get alot faster of a response. also when problems occur, i act as an advocate for my clients to get a resolution to their satisfaction. I am not sure you will get that from booking online.
usually my clients give my contact info to their guests and they contact me with questions about: passports, travel documents that they may need for a minor, activities in the area, special requests that they may have, payment plans...these are services you will not get booking online so why would you if the rate is the same or lower with a TA.