Switching a water heater completely off (or below Vacation setpoint) is an invitation to grow nasty things in your tank, like Leigionaire's Disease.

To kill legionella you need to water at 140 F (60 C) or over, for more than 30 minutes. That temperature will cause a full thickness ("3rd degree") burn in less than a second in children and old people, and less than 5 seconds in adults.

It's best to set the water temperature to no more than 50 C. This is still potentially harmful, but takes a bit longer.

Or set the tank temperature high and have some safety feature at the point of delivery.

Scalds hospitalise many people, killing some of them. Legionella, not so many.

EDIT: It's hard to get legionnaire's disease. The "attack rate" is less than 5%. http://www.cdc.gov/legionella/clinicians.html

We're talking about people that turn their water heaters down during inactive periods or completely off because of a "smart" system that tries to save electricity.

Legionellae grow from 68 to 122F (20 to 50C). If you're letting the temperature of the tank drop into that range for a period of time and then running a shower before it's fully heated, you run the risk of aerosolizing the bacteria and contracting the disease.