One of the main reasons we (PP) request external geometries for commercial sets is to protect you, the creator.

With un-parented props - if a third party does a texture expansion set for them then they have to have textured pp2 files (pz2 files only work on parented props I think - not sure about later versions of Poser). If your geometry is external then whomever buys the texture set has to have your original set to use them. However if the geometry is not external and in the actual pp2 file itself then the textured pp2 files will also have to contain the obj data which would mean no one would have to buy your set.