If x11-apps/xauth is not installed, X11 Forwarding will fail. You can installing x11-terms/xterm on a system with no other X software without pulling x11-apps/xauth into the dependency graph. This causes a runtime failure when you attempt to forward xterm.
Should work without xauth if you disable access control on the server.
nothing to do with bsd...
I'm not sure what you're suggesting. Install xauth as a dependency of any X package? That seems awful. I think comment #1 makes the case that it's not a required dependency by any means.