Distilled water does prevent mineral deposits since it doesn't have minerals dissolved in it. It also offers better heat transfer then coolant/water combination. But it offers NO rust protection and using it in the long may tend to decrease radiator service life due o rust formation and perforation. Water also has a fixed boiling point at 100 degrees Celsius which upon reaching that temp tend to vaporize and escape through the coolant reservoir route. This would lead to decreased water level level in radiator in turn leading to decreased heat transfer which may ultimately lead to early overheat specially in summer months. The coolant/water combination prevents this water loss by increasing the boiling point above plain water. It also has rust inhibitors that will prevent rust and prolong radiator life. It's somewaht a trade-off but it's there are more avantages on using radiator coolant/water than plain distilled water alone. Remember to use distilled water when also using coolant. Goodluck.