Samiel
One Thousand Club
Actually I believe the Half-caste had appeared as cameos for quite a long time before the Players Guide. And Beastmen were always there.Because I like the 1st Ed corebook idea of "celestial exaltation can't be past on through bloodlines; it's too big".  This whole "half caste" thing is just to cover up for poorly explained gods/mortal breeding...
The phrase "Celestial Exaltations cannot be passed through the blood" is quite right and has never been undermined by the Half-caste or otherwise: the half-caste are mortals with a touch of essence in them because of their parents' overwhelming power, and as a result they can learn some basic charms if they work hard to unlock their potential.
In effect, half-caste are like talented mortal thaumaturges: thaumaturges can learn terrestrial MA charms (does anyone know where the cost for these is though?), and can master control of their essence to power artifacts and to use hearthstones as if exalted. Half-caste can do the same, albiet easier, and they can learn charms of their parent's type.
If you ask me, they are the most interesting character type in the game, and given the prevelance of supernatural beings in creation I see them as quite common as well. Their diversity and their half-humanity are an excellent combination for interesting NPCs as well as interesting players.