Back in "the day", heater cores and radiators were brass, and engine blocks, heads, and intake manifolds were cast iron. Ethylene glycol, (green) coolant protected the cooling system against rust & corrosion just fine, but it needs to be flushed annually.

Newer vehicles use aluminum heater cores, radiators, cylinder heads, intake manifolds, and in some cases, engine blocks. Ethylene glycol does not protect aluminum against corrosion. Dexcool, and other variations of coolant, were designed specfically to protect aluminum. Flushing every 5 years/150K miles is too long, it needs to be flushed every 3 years or 36K miles to effectively protect the aluminum components. A 50/50 mixture of Dexcool and distilled water is recommended for the best protection.

Ethylene glycol has silicate in it, (an abrassive type of sand). Dexcool does not contain silicate. When GM started using Dexcool in 1996, the seals in the water pump were redesigned to a ceramic material. Silicate will destroy the ceramic seal, and the pump will leak.

Cooling system neglect, using tap water, or the addition of any amount of ethylene glycol, will turn the coolant into a brown sludge.