Are you sure that Shapeways as it is currently set up is primarily aimed at commerce? And that designers come to SW just to have their designs printed?
It seems to me they have built a space where people can bring their designs, have them printed, show them/share them with other people, sell them, discuss them and all things pertaining to them. Other people who have designed nothing can come and look, buy, discuss etc, and soak up the whole 3d printing vibe.
Perhaps you should compare and contrast what SW have done with their competitors' offerings and see how you think they stack up? There are other sites which offer to print your design for money and perhaps allow you to sell that design to third parties through them, but to my knowledge none of them offer all that is here. It is perhaps a cliche, but there is a community at SW and that adds a lot to the process. Yes, SW is a company offering a service for money, but I would be surprised if their business model was based purely on how many FUD copies of the USS Enterprise they can shift and not on some broader 3D Design/Printing/Maker type play, for which they have positioned themselves beautifully.
To go back to your point, yes these designs
could be shown somewhere else, but why
should they when SW has created this marvellous place? Why have the hassle of your own site when you could use someone else's for free, especially when you know that like-minded people already visit that site?