Cartridge bodies lead to resonances (unless they are properly dampened...), but bodiless cartridges (as Sean said) gather dust and dirt, that accumulates on the most sensitive parts, and impedes the quality of the performance. Research on cartridge body enclosures has led to two results: 1.) large bodies increase bass "volume" (Just as a loudspeaker needs a large housing for a fulminating bass-reproduction, so does a cartridge); 2.) different materials and shapes affect bass "quality." Also, one point that has not yet been mentioned, a body protects a cartridge from electro-magnetic interference and often leads to a quieter performance. There are superior MC cartridges where the body is a crucial, thoroughly-researched part of the design. For example, Belgian cartridge specialist Jan Allaerts produces a cartridge line where the same body is used in the construction of 4 radically different MC cartridges. They are all U-shaped, made out of a specially-finished aluminum, plated with 24 karat gold foil, and coated on the inside with a special matting. The form is said to simulate the Faraday box, and results in a more explosive bass, a quieter background. Lastly, as cornfedboy points out, the differences between body materials results in important sonic differences, but so does the magnet type (alnico, neodynium) and shape, coil winding material (copper, silver, gold), stylus material (aluminum, boron, sapphire), shape, radius, stylus rubber (Shore measurement), yoke construction, etc.
