Advantages
Without a vang you are using one control (mainsheet) to determine two aspects of the sail's trim (angle of attack and twist/leech tension). This is much less precise than having a single control for each, and makes it more difficult or impossible to get 'perfect' trim. If all you want to do is make the boat go from A to B, then you don't need a vang, but if you are interested in having her perform to her potential and in learning how to trim the mainsail efficiently, a vang soon becomes a desirable piece of equipment. There is no difference between good cruising trim and good racing trim - they both indicate that the skipper knows how to shape the sails (and a vang has much to do with that.) A rigid vang is especially nice, because it takes over some of the duties of a topping lift and will support the boom when the mainsail is dropped. Once you have sailed for a while with a vang-equipped boat and know how to use it, you will feel the limitations of going back to a mainsheet-only setup.