Out of curiosity I've taken the cable to my office this morning. Here I have a good old Dell PWS 360 with a Quadro FX 3000. It plugs into the card without any hassles. So I guess this cable is for optimized sync for professional applications.
Thanks for your answers. So 3D vision 2 works without this cable and head scratching has ended
Nevertheless, I'll try to get a cheap Quadro for home to test if there is any difference with and without this cable.