I think Mr Mclellan offers an interesting comparions here. There is one missing point though. The Adobe solution is compared simply to 3D viewers. If you look at the usage of PDF as means of communication, the Adobe solution is much broader than simply another 3D viewer. Here you have the ability to combine 3D object with other elements of a document for a much broader reach.
3D visualization has a great deal of value and is certainly a needed element in the design and manufacturing process today. However, viewers tend to be just what they say they are. A means of viewing CAD files without the original CAD application with some additional (and variable depending on the viewer) functionality that allows for degree of analysis.
Being able to combine 3D CAD objects with other elemnts of information and allow anyone with Adobe reader to use and interact with it puts 3D visualization on a completely new track for a very large audience.