The pen is mightier than the spreadsheet

Brian has just blogged about a burn down chart, he says:

"My chart differs from Kelly's in that I removed some extrapolated lines he uses. I like to get away with as few predictions as possible. To emphasize that, I hand-drew the one prediction. That makes the line seem less authoritative and believable than one Excel draws, which is appropriate."

When your sponsors are making financial decisions based on your reports, you want to make it clear that the predicted end-date is just a guess, not a calculation.
This concept has some similarity to the idea of fuzzy vs hard analysis and lo-fi prototyping, and is probably even more important for project reporting than requirements gathering.
Incidently, this is why I dont like function points and other calculated estimates, I prefer a hand written number on an index card: it is clear this is just a gut feel. It makes it much harder to leap to the wrong conclusion (that reality will somehow match up to the plan).