In the books, vampires completely drain their prey (human or animal) of blood when feeding, or at least as much as they can get. The times when a human isn't sucked completely dry are usually when the vampire is interrupted in their feeding (as in Carlisle's case; also, most of the scenes of the newborn army in Eclipse involve newborns breaking away from feeding to fight their fellow newborns), the vampire is trying to turn and not feed on their victim (numerous examples), or the vampire is simply trying to kill (Rosalie, killing her former assailants after she was turned).
To turn a vampire in the Twilight universe, you cannot drain them dry (unlike many other universes where draining and then an exchange of blood is required). So, when you're actually trying to create a new vampire, you just have to bite, but then you have to control your thirst and not feed on the human you're turning. Of course, draining a LITTLE blood is OK.
When a vampire really is interested in feeding, understand that a cut to the neck (or any major artery) only produces blood as long as the body continues to pump it, thus producing blood pressure. Once the heart stops and the blood volume is no longer "stretching" the arteries and veins, the bleeding would stop, and it's rather difficult to "suck" blood out of a body through any extremity's artery including the carotid. Only gravity would continue to produce blood from the puncture, and we don't typically see vampires stringing their prey upside down to let the remaining blood drain. So, a "drained" victim (no longer pumping blood into the vampire's mouth) can still bleed after being discarded.
Also, from an out-of-universe perspective, any demonstrated "wastefulness" on a vampire's part simply increases the horror factor. Not only is there more blood to show pumping out of neck wounds, but the victim is often still alive, and the vampire often tosses one victim aside to sink their teeth into another. As the Twilight films are PG-13 there's a cap on the amount of gratuitous blood that can be shown, but you can still have a little fun.