If you model a radial nurbs characters head for example, it's build up from front to back and gives you one continuos surface, easier to texture but harder to model because;
When you model in patches (also nurbs btw) you have control over what areas have more detail and isoparms, you stitch them togheter later on...or not, some people solve this with different techniques.
When you trim surfaces you make a curve for, say the eyesockets and cut that out of your head. At the place of the cut you create a new surface between the cut and the eyelids.
You can read a lot about these subjects at the Gnomon site.
Hope I clarified well, Maos.