A lot of people out there don't have access to a 3d printer so they are willing to get something printed through a 3rd party like Shapeways and CG Trader if they find a unique design they can't find anywhere else. Price still is a problem through using 3rd parties to print a design as designers don't make much profit. Cg trader has some of the highest prices I've seen for their "Get Printed" now feature. My Chibi Alice costs $56 to get printed through CGtrader where as on Shapeways it costs $19. I make more of a profit on shapeways than here on CGtrader when It comes to having a 3rd party company print a design but I make far more profit when it comes to selling stream/downloadable files for printers.
As 3d printers become more cost friendly for the average consumer, the demand for quality streamable/downloadable printable files will go up. Stream vs download I see as as a own vs not to own. If a buyer as no intention to print multiples of a files, streaming may be the way to go. If a designer allows downloads of a design, the buyer can modify and print as many copies as they wish. And like anything digitally downloadable on the internet, doesn't guarantee it will be protected from illegal uses.