The GM instructions do this in part to cover their butts, less advance means less chance of detonation/pre ignition due to over advancing and thus fewer warranty issues.
Learn the advance curves of you distributor, how much mechanical advance does it add, how much vacuum adv does it add. There are hundreds of threads on this. Set them up to suit the engine/truck
For the most part it is advantageous to run vac advance as it can help improve part throttle effiecencies (fuel milage as vinnie states), clean up idle characteristics etc. some motors can benefit others not so much.