While I agree that this would be nice there are some issues that can come up. Multiple sessions of ArchiCAD can jam up even the most powerful machines (especially with multiple OSs running as I often do) and the present setup (besides easing/enabling the multi-sessions on the Mac) also fixes problems that we used to have when Windows would launch multiple sessions by default.
The .lck files may have fixed this as well but I can recall clients on Windows who would unwittingly launch multiple sessions of the same project by double clicking the file in Windows Explorer. Kind of a beginner mistake I know, but a big problem for anyone who did it and GS has to keep these sorts of things in mind also.