Donated blood is edible, and even at room temperature and full of whatever it is the Red Cross puts in it, it's tastier to vampires than animal blood. Doctors, such as Carlisle, are legally able to purchase blood, and in fact he is shown to do so in canon; no concern about depleting the blood supply is mentioned, although it would be surprising if there were none.
However, consuming human blood instead of animal blood makes vampires less able to resist the impulse to eat live humans, who are yet still more appealing than donated blood, even when the vampire attempting to resist is not particularly thirsty. The thirst is constant, and while it is reduced in force and intrusiveness by having recently eaten, the factor preventing the Cullens from slaughtering everyone in town is willpower, not disinterest. Edward is shown in "Midnight Sun" to be able to eat half a herd of elk and come to school the next day and find that Bella's scent is "like a wrecking ball", no less demanding than it was the first time.
As a much more minor side point, drinking human blood makes vampire eyes red, which would be more conspicuous than gold, black, or any of the range of colors in between. Contact lenses are possible, but are dissolved over the course of a few hours by the venom in their eyes.
Why don't the Cullens keep a stockpile of stored blood (human or animal) on hand to use in crisis situations? If they had, then Edward would not have had to leave Bella alone to go hunt in chapter 23.
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website <strike>(warning: white text on black background)</strike>.
Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
Spoilers for the fanfic itself as well as the original novels need and should not be hidden, but spoiler protection still applies for any other works of fiction, except for Harry Potter and the Methods of Rationality chapters more than a week old so we can freely discuss similarities and differences.
EDIT: Post-ginormous-spoiler discussion should go to the second thread. (If you have any doubt on whether you have reached the spoiler in question you have not.)