Even though the system is pressurized, I have to imagine that a 50% coolant level means that certain parts of the engine aren't being properly bathed in coolant, and hot spots would develop. While perhaps this may be less pronounced at lower ambient temperatures, I hope that you don't need an operating heater or defroster, as there is no way that the heater core, fed with relatively high-mounted hoses, will consistently flow coolant at such a low level.