Okay, so I'm slightly warming up to this idea. Since it's so uncommon and difficult, it would make sense for Luna to be a natural born vampire, and play into Angel's story about half of her family being vamp, and the other half human, simply put- it is impossible for two human parents to create a vamp as is two vampire parents. Only the mating of one human and one vamp could create a child. It's so rare, that Luna was the last one raised to adulthood, simply because of both their innate power and their ability to bring both created vampires and baby vampires into this world.<br/><br/>It also explains Angel and Luna, in that they are of the same bloodline. Luna would likely be Angel's great-great-great-great aunt, and if there had been a point where Angel's family had been overrun by newborn vampires, much to the detriment of the Vampire side of the family (AKA Luna et. al) there would be sadness and tragedy there. Enough that Luna would seek to not be reminded of her every millennia hence Angel is knocked out and made to rest.<br/><br/>It also means that there is a reason for the Prince to seek power from Luna, if he had already tried to create an heir of his own and failed.