What if (This maybe insane) What if EMS makes your eye so powerful that the user can create the MS jutsu without activating the 4th stage. Madara uses a his space-time jutsu, it clearly comes from his sharingan eye. Madara is the only confirmed EMS user to have gained EMS.
The purpose is your only using half the eyes power. That's also why he uses the jutsu and it looks like he's not losing any chakra. It would make so much since as to why EMS is so great. If Naruto faced Sasuke, Sasuke would be dead in the water after about 10 minuets.