I surely don't understand why serpent=evil only!?!
(Maybe because this idea has been going since the story of Adam and Eve in the Old Testament, but IIRC the serpent was related to the Tree of Knowledge of Good and Evil in the Garden of Eden. But it was also reverred as wisdom and related to the Night, only when the Sun took the place the Stars, Night and Moon the serpent was portrayed as evil)
Sure it is a no brainer for game designers to do this, but with the change in alignment and mostly unaligned people, there could be a fragmented population leaving the underdark to adopt borderlands, if they are intelligent and fine manipulators there could be some spies disguised as humans, and there could some setlements that are influenced or attracted by civilization and be considered as traitors by their fellows, but the other races could see it as an opportunity and be a little more accepting toward the Yuan-Ti than before. They maybe would not be trusted by humans, elves, eladrins, dwarves, dragonborn and halflings, but they could mingle with the tieflings, the gnomes, the kobolds, and the gnolls.
Anyway, this could make good campaign material, and additional twists to the already existing new PoL 4E world.