Except not all the girls had a choice. And just because Buffy's setting up a sisterhood cult of Slayers and they don't have to be alone doesn't necessarily mean having your body metaphysically altered is going to send you skipping down the lane. Some ladies might have serious problems with that.
What if you're a Quaker and the Scooby gang shows up telling you it's your destiny to kill demons?
What if you're a devout Buddhist and this breaks your code to respect all forms of life (which presumably would include demon life, too)?
What if you just don't wanna and you run off to Canada (hai, urania_calliope, can I crash with you till the apocalypse stops beeping me)? I mean, isn't being Chosen by this spell but not wanting to fight kinda like avoiding the draft during Vietnam (except the draft means you're made into physically "trained" with instant-warrior-magic)?
So, a Quaker, a Buddhist and a hippie named Flower (she comes from a long line of hippies) walk into a Slayer bar... stop me if you've heard this one before...