If you are going to build something, why not do it right. Make it withstand extremes because we will always have extreme weather patterns so might as well be prepared. Do it right the first time.
I don't necessarily disagree with that, but (in my opinion), you have to operate within the normal distribution of risk.
Accounting for outliers (like this) are incredibly expensive. If you want to protect against them, ok fine. But be prepared to pony up the money.