Voted up for excellent points all around.

I have in fact completely given up on giving probability estimates more extreme than +/-40 decibels, or 50-60 in some extreme (and borderline trivial) cases. I haven't actually adjusted planning to compensate for the possible loss of fundamental assumptions, though, so I may be doing it wrong... On the gripping hand, however, most of the probability mass in the remaining options tends to be impossible to plan for anyway.