Think about it, the way it's symbolized, it looks like Twilight and the other Alicorns faced a threat so great they gave up their powers to create the crystals, giving one to each leader of their respective race. The crystals are useless of course, unless all three pony tribes are friends, if not they are just crystals. Sunny was gifted with Alicorn powers (while not becoming one) my theory is, it's kind of like whoever wields it, is the rightful ruler of England) the power would allow the pony who wields it to represent all races of pony, while not changing into an alicorn. I'm convinced that Alicorns are extinct, or like one theory said "Celestia and Luna were not really ponies at all, but incarnations of the sun and moon themselves, and eventually became one with them again. That's why they don't have a mother or father and why there are no born alicorns(apart from Flurry Heart)