And without raising awareness of the issue, everybody might always be somewhat vulnerable forever, whereas now that "we know", after being highly vulnerable for a short time everybody's vulnerability to this should drop to zero very quickly.
If you assume a limited number of evildoers and a limited ability to exploit this at will (e.g. you have to catch your victim in close proximity on public wi-fi that you're sharing with him), releasing a tool like Firesheep may produce significantly less total damage.