You are merely wrong in calling the system gravity feed/syphon. Yes, if you drink the water from the hose when the system is charged(water in the hose), and the end of the hose is below the water level in the bag, it is as you described, gravity feed/syphon. But because you bring the hose up to your mouth which is above the water level, it loses this function, and it is up to you to suck the water against gravity. There is no pressure in the system unless you physically squeeze the bag to add pressure. The mouthpiece is there to either stop the water from running out if the end hangs below water level, or if it hangs above, to prevent the water from running back to the bladder so that you have to suck the air out of the hose on your next drink.