I've used Xenforo since 2012. In all that time, I've only used this function a few times. And it was not just based upon user, but on an IP. Of course, the user apparently was not smart enough to know about TOR or willing to get a VPN. It's an OK feature, but easily gotten around. Users that are aware of it will just create new accounts. If it's IP based then they will use a VPN or TOR to get around it... unless you block TOR at the firewall level.