Although he was talking in a very different context, I still think that Bruce Schneier's perspectives on worst-case thinking have relevance to us:

"Worst-case thinking means generally bad decision making for several reasons. First, it's only half of the cost-benefit equation. Every decision has costs and benefits, risks and rewards. By speculating about what can possibly go wrong, and then acting as if that is likely to happen, worst-case thinking focuses only on the extreme but improbable risks and does a poor job at assessing outcomes." (from Schneier on Security)

I recommend you read Bruce Schneier's perspectives on worst-case thinking, it's quite interesting, and you will see his second and third reasons why we need to be careful with worst-case thinking.