If we were to extrapolate your concept of 'standby' or emergency' service governments could save a fortune by killing off fire and sea rescue services on the grounds they are only needed every so often.
Perhaps you are unaware that many public works, including roads and the Thames Barrier (another waste of money using your criteria), are designed to survive any historical challengers that have occurred in the previous 50 or 100 years.
Another example of risk input into capital expenditures have occurred into response to recent rail/tube accidents. When 31 people were fried to death at Kings Cross on 1987 November 18 as the result of a fire on a wooden escalator, a subsequent report recommended all wooden escalators be replaced with metal treads. The recommendation was rejected after cost was considered including the cost of compensating for any deaths - figured at around 2-million pounds per body.
The present Heathrow has been at it's present location since New Years Day 1946, it was first Fairey’s Great West Aerodrome then RAF Northolt, and if BAA has it's way it will be there forever.
This means (1) BAA has to take into account historic conditions; (2) The level of service it proposes; (3) Capital investment needed to achieve these goals.
Even if an event occurs every 20 or 30 years BAA has to take it in to consideration. Notwithstanding BA's attempts to increase the accident count (2008 BA flight crash-landed) BAA still maintains a very expensive fire service that spend months, even years, waiting for the next crash.
The provisioning of this fire service, and even snow removal equipment, is based on historical demand, and proposed service levels. BAA claims Heathrow is a 'world class' airport, it doesn't claim any exceptions. Therefore it has failed in it's duty. It matters not an iota whether the interruption is caused by a humans or Nature exercising it's options.
Talking about 'whingers', you might want to canvas the opinion of those people sitting it out in the airport. They might have different opinions to yours.
The cost of capital equipment, amortised over 50 years or more, might look like a real bargain when stacked up against airline costs (including repositioning empty planes), BAA losses and added costs, passenger losses and added costs and losses to the general economy.
Only people with a myopic viewpoint can't see why BAA failed.
P.S. Where I am has a temperature of 35c, the sun is shining and the drinks are long and cool. But the BBC World Service TV makes us appreciate our luck!