Woodcraft puts that on their training descriptions. "Bring a sharp
gouge". I asked them why once, and they said that if they didn't say
that, people would bring dull ones.
Maybe it's because chisels and gouges are tools where the force on the
tip is generated by (or controlled by) your hand, instead of a motor?
Therefor, it makes a difference if it's dull - a motor can force a
dull saw blade through wood, but it's much harder for your hand to
force a dull chisel through wood.
Aren't dull chisels used for scraping paint, anyway? ;-)