I use an agent. Some prefer to do it themselves. But looking beyond the comparisons of service, advocacy, etc., the simple fact is that booking on-line is not always cheaper than using an agent.
An agent could have blocked out group space for a particular cruise, and can sell it for less than what on-line booking agencies can. Case in point - our recent Alaska cruise. I had already booked it. My agent saw lots of interest in that cruise so she reserved group space, moved me to the new group booking, and as a result my price went down and I also got an onboard credit. Do you think an online agency would automatically do that? Think again.