how important is coolant?

[XTO]

XTO: That's not exactly true, while cooling capacity may be close (similar, due to pressurized system), water alone will not provide lubricity to the water pump.

Common sense lang naman, car manufacturers wouldn't REQUIRE all cars to be filled with the proper coolant mixture if it's not optimal.

People tend to draw their own conclusions din kasi usually based on lowering cost alone.

yes it is a common sense to create our own conclusion based on our own observation and experience.