I had assumed this was like Adobe Acrobat and Reader. Until I needed such functionality and actually downloaded Reader and Explorer. (v10)
Unless ESRI adds ability to export to .pmf in ArcMap, and/or import .mxd in Reader and Explorer... it will significantly limit the usefulness and number of users for Explorer and Reader. Adobe sells Acrobat because everyone has Reader and thus PDF become the de facto document standard. Same for AutoCAD .dwg and Design Review. Perhaps if ESRI opens up MXDs it will become the de facto map document? Food for thought. I'd bet it'd make more money in the long run than a handful of Publisher licenses. 😉