The Character Diamond enables you to create colorful and fresh characters. Season them with a few quirks and the characters develop even more uniqueness. They may become so unique and so unlike us, however, that we can't identify with them. If we don't identify with them, we've lost a major opportunity for creating emotional immersion in the game.
It's worth reiterating that NPC Rooting Interest Techniques are just an other Emotioneering tool and not necessarily one that benefits every game. For instance, if you were creating an urban game where you were alone in a hostile city, fighting and killing everyone you encounter, then you probably wouldn't want any of the NPCs to have Rooting Interest.
Even in this situation, however, it might be worth asking the question if it wouldn't be worth changing the design of the game and using Emotioneering to create greater emotional engagement by the player. In this example, I might ask the developer if the game wouldn't be improved by having a few NPCs in the game who are more than cannon fodder.
But how do you create Rooting Interest for an NPC?