and for another reason... you are NOT given access to the questlines (backstory) associated with either vampirism or being a werewolf, that you otherwise get to experience with the bites. There are areas of the game that you will not be able to otherwise access if you purchase the bite.
If ZOS would add access to the proper questlines with the crown store botes I would not campaign as hard against them... Until such a time GIVE THEM NOTHING!