combustion chamber pressure is at about 100 magnitudes more than your cooling system, any leak exposing the cooling system to any combustion chamber will always result in combustion gases in coolant which will cook the coolant really bad and overpressurize the system (the radiator cap will be blowing coolant into the overflow tank violently)
coolant has a task to lubricate the water pump seals too - and there is a controlled leak built into the design of all water pumps just for this purpose. A leak of about 1ml every 100 hours or similar is considered normal (I can get you an exact figure if you like) - that is why you see a trace of colour at the weep hole.
using plain water is hard on the water pump seals as it has no lubrication properties at high temperature.