SailorJack, with me it just depends on the trip. For my upcoming Baltic cruise I used an agent since it involved international air travel, transfers, travel with an elderly parent, as well as wife and son, so on and so forth. There are lots of details and I would rather let her, as you say, do the heavy lifting.
My other two cruises I just did myself on-line; one involves no air transportation or transfers and the other a simple one-way flight to Ft. Lauderdale which I booked through Princess. I've traveled enough with Princess that I don't really have any questions, or if I do, I know where to find the answers. The agent just seems like an unnecessary extra layer to get through. And I found out that once booked through an agent, the cruise line is not responsive to problems; they want you to go through the agent, which I understand but I don't like.