It's not a fundamentally bad idea from a practical point of view (like I said, I'll leave the ethics out of it), but I think it is not as good an idea as a lot of people think as it narrows the field so much. Also the signal that people get, unless they have worked directly with a person, rather than just chatted with them at a networking event or know them from their childhood, is I would argue "belongs to my tribe", rather than "good at x"