Ford has paid me good money to examine head cooling passages from cars that had peculiar conditions during warranty. A few of the heads investigated were abused by people who drained the factory coolant and used straight water, typically truck owners in Texas who want to increase thermal transfer without realizing the downside of such action. The nucleant boiling from using straight water cavitated (pounded holes) the inside water passages until it reached the outside of the head, resulting in a leak and total loss of coolant.

A 50-50 mix (and proper 15 psi pressure) will raise the boiling point to where you don't get this phenomenon. Whoever told you the story got it backwards. The heads for repair he mentioned were the result of using straight water, not the reason for using straight water.

__________________
95 E320 Cabriolet, 169K