It doesn't quite ring true for me either, but who can say what ideas get passed back and forth when seasons get planned out? Faith's character in S3 is so much rooted in the idea that being a Slayer makes her able to do what she wants with no consequences that I can't see how it would be a natural development of that.
Perhaps the real potential of her character didn't really come out until she appeared on screen and came to life, but the life that the character has subsequently had shows what a waste it would have been.