Just a little clarification, 

1. Coons surface - there doesn't actually need to be the same number of drawn/clicked points per side when drawing the guidelines. When you complete the last line selection with the Coons tool, Carrara automatically adds extra points to create a quality grid structure. This is true whether using the interpolated line tool or the simple polyline tool.

2. Gordon Surfaces - like the Coons shape, connecting longitude and latitude lines MUST connect. The only way you can do that is by holding the shift key down while clicking the line tool on a point. Close isn't good enough. Even if one point is not connected, the Gordon function will fail.

The Coons tool is excellent for hoods of cars, airplane fuselages, wings and more - especially when creating them from plan views.

The Gordon tool is great for asymmetrical full shapes like boat hulls and shapes that have multiple compound curves. The image is from a project where the Gordon tool was used for a boat hull.

Hope this helps.

Mark