A piece in TechCrunch examines the notion that common availability of 3D printers will give rise to a "Napster for 3D Printing Models".
Napster, for those of you to young to recall, was the first big-time music sharing service, which emerged at a time when rules about such things were vague and corporate responses were non-existent. Today things are much different.
There's already a small trade in physical 3D models, piggy-backing on services and technologies previously (and still) used for sharing music, software, books and videos.
Sharing of these items today is a different experience than shopping in a store: finding items can be tricky, media quality is often suspect and it's usually illegal. Shared 3D models will suffer the same problems - and one more.
When a song or movie has "degraded quality", it's usually still watchable or listenable to varying degrees. Sure, it may not be as clear, but it still functions for your purpose. Meanwhile, 3D models of functional objects may not function at all. Quality problems in a 3D print could be catastrophic.
Consider this: download a Mastercraft wrench model, print it and then have it break the first time you use it. The materials you print with may differ from those engineered into the original design. Your 3D printer may not produce the quality and robustness required for the object to work properly. Finally, many objects are made from more than one material - something very few 3D printers offer today.
If there's to be a Napster for 3D printing, it will be for very simple non-functional objects. We'd get worried when personal 3D printers are far more capable then they are today.