A normal lens is much more efficient than a sphere. And a Fresnel lens is more economical as well as more efficient.
It's very pretty, and it may actually be intended as a work of art. But it's not efficient technology.
Here is an optical analysis of a sphere used as a lens -- the light doesn't converge to a single point, which is why spheres are more decorations than lenses:
http://i.imgur.com/4e4FX.png
