It is supposed to be a closed system under pressure. Think of it this way, the greater the difference between the coolant and the ambient air the more efficient the cooling system. Water boils at 100C at one atmosphere of pressure but at higher pressure the boiling temperature is higher than 100C so the cooling system is more efficient. That's why a pinhole leak in a cooling system is a problem. Not because it leaks fluid but because it depressurizes the system causing the coolant to boil. The coolant reservoir is meant only to supply coolant to replace lost fluid when the bike cools down again and the pressure in the cooling system becomes slightly less than atmospheric. This allows the system to replenish its coolant without sucking air into the system.