I understand your point. However, I’m not using tags in Agenda much because the interface doesn’t support them so well as it does categories and projects right now. There’s no autocompletion, there’s no browser, they can only be assigned via editing the text of the note, there’s no mechanism for batch tagging, parameterization is mostly just a textual convention, there’s no nesting, tag names can’t have spaces, you can archive a project but not a tag, you can link directly to a project but not to a tag, etc.
Also, you can tag notes, but you can’t tag projects. This makes tags tedious as a means of identifying membership in areas of responsibility or focus. Projects typically belong to one or more areas. If tags are used to identify areas, and a project belongs to one or more areas, then every note in the project needs the same area tags. That’s where membership in multiple categories would be useful for projects.