You are correct with the two different froms for 2D vs. 3D. The reason for that is the 2D data (in some sort of xml format) is readable by the Microsoft XPS viewer which is included with Windows, so basically any windows (Vista +) computer can read 2D DWF data without any other program installed. This ability is limited to the DWFx format (which is kind of the new defacto).

The advantages of DWF over pdf are file size, and a more robust 3D viewer in addition to free markup tools (Adsk Design Review). DWF really is a more CAD-centric format than PDF. I *think* 3D pdf was kind of a reaction to DWF and is now in the hands of Tetra 4D (?) instead of Adobe. It's a good way to hand over a design for review without actually handing over the design. There aren't any specific programs (besides Design Review) that only accept DWF that I am aware of.

It has it's uses, but is not going to be a "Wow, I'm sold!" feature.

I attached a quick demo dwf (just some basic shapes). Note: only 2D stuff will show in xps viewer.