helidrvr,
You are right, a good twin with one shut down is just like a single from a failure probability, so what's the worry?
Simple, in the case of the single, the pilot had no choice. In the case of a twin with two good engines, when you shut one of them down, you earn the right to explain to every Tom, Dick and Harry why you did it, should Murphy show his hand. Trust me, after you land, you will meet experts who never flew higher than where the top button of an elevator could take them, all quite willing to explain your mistake!
Choices, always choices!
Nick