There's evidence that US government counter-intelligence and information security are very poor: Snowden, Manning, the recent break-in at OPM, etc. In past generations, nuclear plans were stolen, the heads of both FBI and CIA counter-intelligence have been moles, a Navy sailor kept the Soviets updated on the locations of US submarines, etc. Here's a list I came across recently: http://www.wearethemighty.com/american-…
Why is secrecy clearly necessary for governments? You may be right, but it's far from clear.
But I think the most common use of it is probably the fact that politics is a game of hypocrisy, and you can't play unfairly if you don't get to keep secrets.