It would be amazing if we could use positive/negative/intersect clipping for making more complex shapes out of simple prims. To place a sphere centered along a planar side of a cube in such a way that the sphere was invisible, but left its "indent" in the side of the cube. Just one example of what I mean. The texture of the sphere would be used to fill in the "indent".
Perhaps this is asking a bit much of the Second Life engine. As I'm thinking it through, perhaps the SL engine merely "radiates" the shape around the prim's epicenter and so would not be able to calculate where the side of the cube and the sphere intersect. But perhaps in the future, if such a thing could be taken into account... there are many objects I'd like to make, but can't given this limitation.
One example of such prim building I've had experience with is that found in Bryce3D, which does have the pos/neg/intersect clipping feature.