Costco is a real
travel agency and I am very glad I use them as I have gotten thousands of dollars back over the years of booking our cruises with them. For me, that is more valuable than having an agent to chat casually about cruises with. I don't need or want that. If I were that chummy with an agent, I'd feel guilty about cancelling or making a lot of changes to a cruise, and I never have to feel that way with Costco because the phone reps are personally unattached to my booking, though of course they are still courteous and professional.
Others can have different preferences in a TA, and that's fine, but there is no need for those people who choose other types of agencies to bash Costco. As you can see from this thread, most people who book via Costco are happy with them.
I have also seen threads & posts by people who's personal T.A.'s from traditional travel agencies ignore them or don't properly fulfill their basic requests, so having a personal T.A. is no guarantee of a good experience.