Not a justification but maybe an attempt at an explanation: planning for the worst case scenario 1) reminds people that the worst case scenario exists, which is uncomfortable, and 2) makes you look suspicious. It looks like you have some inside knowledge about how likely the worst case scenario is and/or are planning to bring it about yourself for some reason.
It also increases the implicit likelihood you will attribute to the scenario, unless you have superhuman bias-checking abilities. In some cases that can increase the actual likelihood, which I guess is your part of your point #2.
Today's post, On Not Having an Advance Abyssal Plan was originally published on 23 February 2009. A summary (taken from the LW wiki):
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