This is the argument that the W3C uses to sell Schema too.
Users range from the data entry clerks and subject matter specialists to the topic map creators.
Been there done that. The existence of a single underlying tagset in XTM has no impact on what is displayed to the user. What is displayed to the user is the choice of the interface designer. As for human verification, I suspect experience will show that even relatively simply topic maps will quickly grow beyond human verification, at least in the sense of verifying the syntax of the map. Human subject matter specialists will be called upon to judge the accuracy of the content of the topic map but that is an entirely different issue.