Bruce Schneier has a typically good essay on the use of "worst-cases" as a substitute for real analysis. I noticed conspicuous use of "worst-case" in early reporting on the oil spill in the Gulf. It conveniently gins up attention for media outlets keen on getting audience.
There's a certain blindness that comes from worst-case thinking. An extension of the precautionary principle, it involves imagining the worst possible outcome and then acting as if it were a certainty. It substitutes imagination for thinking, speculation for risk analysis and fear for reason. It fosters powerlessness and vulnerability and magnifies social paralysis. And it makes us more vulnerable to the effects of terrorism.
Worst-case thinking---the failure to manage risk through analysis of costs and benefits---is what makes airline security such an expensive nightmare, for example. Schneier concludes:
When someone is proposing a change, the onus should be on them to justify it over the status quo. But worst case thinking is a way of looking at the world that exaggerates the rare and unusual and gives the rare much more credence than it deserves. It isn't really a principle; it's a cheap trick to justify what you already believe. It lets lazy or biased people make what seem to be cogent arguments without understanding the whole issue.
It's not too long for you to read the whole thing.