Robert,
True, referencing an obj won't protect you from people illegally distributing the file, as you've stated. Heck, they could just distribute the obj file. What referencing the obj does is give people the opportunity to legally distribute pp2's with morphs and such for your prop (if your selling the item, this could be good for business). It's certainly not a requirement, and not as necessary for props as it is with character .cr2's, but it could be worth the effort. Plus, if people have the seperate .obj file it's easier for them to make morphs in modelling programs and re-map the .obj for texturing purposes.