NURBS are mathematically accurate surfaces, but are harder to create intricate models with. NURBS and polygons both have their pros and cons.
From what I understand, it’s impossible to convert a poly-mesh into a NURBS mesh, so the 3D Warehouse would be useless if SketchUp converted to NURBS. If it was easy, I’m sure SketchUp would’ve solved the problem already. From what I’ve read,
I agree that vector-esque curves would be amazing though. At least the polygons are vectors, rather than voxels or something. 
Ideally, I’d like SketchUp to combine the upsides of NURBS and polygons into a single tool, rather than making a NURBS toolset and a polygon toolset. The circle tool should make a real circle and you should be able to make the model model as complex & artistic as you want. But that might require some genius-level innovation because there might be nothing like that available yet… If NURBS software is complex to make, just imagine how complex it’d be to innovate a new type of 3D modeling software.