Do NOT move folders that are inside “Runtime:Libraries” but outside of the special folders (Character, Camera, Light, Hand, Face, Hair, Pose, Props, Material)—they need to stay where they are.
For folders inside those 9 special folders, it depends on what application you’re using, In DAZ Studio or Poser 8+ you can move folders from one of the special folders to another. In earlier versions of Poser specific filetypes have to stay in specific special folders.
To be fair, much like Poser, Daz Studio can browse through the libraries folder much like you’d browse through it on your hard drive. Even creating new folders is rarely a major issue as long as important files aren’t moved like the geometry and the textures. Essentially this means that the library folder can be tailored to suit the individual.
This isn’t true for anything with associated metadata, of course, but the majority of Poser format content which is stored in Runtime\Libraries doesn’t include this anyway.