Nurbs are made of curves, while polygons are created using points. In order to maintain curve geometry, you aren't able to simply delete a surface patch with Nurbs. You have to manually cut along isoparms to maintain the 4-sided geometry that is required for a nurbs object to exist... hence the need for patch modeling when using Nurbs. With polygons, because the geometry is being constructed with multiple point coordinates, deleting a single (or multiple) face(s) doesn't deconstruct the entire object.