That makes more sense, but it’s still not a good idea. The simple reason is that it’s putting a weapon in a place where she can’t actually control it. The problem is the assumption that criminals don’t talk, that people don’t talk, that policemen don’t talk. Someone wears spikes in their hair, they’ll start building a reputation, once that reputation becomes more public then the people she faces on the street will know and they’ll start using it against her. All any ruffian on the street has to do, is ensure that they tangle their fingers up in her hair in the exact right way and then jab the spikes into the back of her neck.
You pretty much never ever want to go into combat with a weapon that you don’t control, especially not one that’s easy to use against you. This is sort of in line with putting a knife at the end of your braid and using it as a whip, it might work for a little while, but in a mad dash scramble it’s just as likely to cut you as it is the other guy. It doesn’t have to be another person, her braid could just get twisted around, or slammed into a wall, or a door, or any other thing and end up in her tissue as it is to hurt her opponents.
You just don’t want to wear anything that can be used against you when you’re going into a fight, because given enough time, other people will find those places and maybe it’s not this guy. But that guy you just put into lockup is still alive to remember, he can talk to his buddies and the other criminals in the holding cell, they’ll know and they’ll remember. Not all of them are going off to the mines, or prison, or a day labor camp.
It’s that sort of fallout that you really need to keep track of with your writing. Pierce is usually better about this stuff and like I said, it sounds like a good idea until you actually try to implement it. If it actually worked, more people would do it. Really.