I think most people underestimate those guys : Prince Taldaram is a very solid card, with tons of applications. Even though its lategame potential is arguably worse than Faceless Manipulator, people fail to realise how much more useful he is in the early game.
Prince Valanar is a neutral heal, so you shouldn't except too much from him. While its raw potential is limited, his drawback is not that harmful for some classes : he is a natural feat for Miracle Rogue (maybe Unicorn Control Hunter as well). Since he fills a role badly needed by those archetypes, he is quite likely to see play.
Sure Prince Keleseth seems very niche : the only application i could see for him is Quest Hunter. Still, classes like Paladin, Rogue or Warlock could make some kind of aggressive deck work with this guy.
So i think those gut shouldn't be dismissed so easily : they sure do look more viable than at least half of the legendaries revealed so far... (compare with Moorabi, Hadronox or Sindragosa)