I think that when 3ed started, there were cool ideas for weird PC/NPC characters to drop in the world. When you read the descriptive text in D&D products it always seems like this sort of things are RARE, just as many monsters also are said to be extremely, so that few people in the world should know they exist... even core classes, or at least some of them, are supposed to be rare: the Paladin being someone who became enlightened to fight against evil, the Sorcerer is said to have dragon blood and is some sort of outcast because of him being unique, the Monk being a strange person coming from a distant exotic culture... The fact that they are rare and "special" is in line with them being PC, since they are the protagonists of the story.
However, once you get used to this sort of stuff, the player's perspective change the world completely, and everyone (the authors first and foremost) suddenly percieves the fantasy world completely full of the most diverse people and creatures.
Personally I think that the gaming group should still be the responsible for choosing in which kind of fantasy world they want to play, it doesn't matter if the book says "there is only one platinum dragon in the universe" but then gives a prestige class tied to a society of half-platinum dragons.
Books should always be used as tools, but never have the right to tell you what exists or not... as such, just consider the half- stuff around as tools or mere ideas, and don't feel compelled to use what you don't like
