Sure, if that's what you guys want to do. I'd make sure everyone was really, truly on board though. In the wrong hands, that kind of thing leads easily to silliness.
How would they inflict harm? Just through biting? They wouldn't be able to inflict as much harm as everyone else. Gunfights are right out. What if they want to take the advancement and become a different playbook? What if they become a driver? What are you going to do about the sex move? Part of the fun of being a brainer, I think, is finding ways to trick people into fucking you. But I guess if they're good without that, then sure. I guess it could still happen anyway.
I mean, as long as you've thought really hard about what this really entails and what the player might be missing, not being a human being, then go for it.