I use both the internet and a travel agent in planning my travel. The best thing a cruise travel agent could do is to ask the clients enough questions to really understand what they are looking for on a cruise. There is just so much variation between the different cruise lines that getting the right line for the customer is very important. It would also be important to explain the differences between the newer and older ships. You should also help your clients avoid bad cabins due to proximity to public areas, etc.
I would also find it helpful if my travel agent worked to save me money. This could be as simply as suggesting that if I took my cruise a week earlier or later that I could save money. I also would appreciate it if my travel agent would let me know when a real "deal" comes along that I might be interested in taking.