Having a dice pool of 'good' and 'bad' dice allows for a unique option when compared to simple D20 systems and similar.
The GM may frequently award fortune dice and expertise or penalize with misfortune and challenge dice. But he also has the option adding both good and bad dice to the pool creating the possibility of some interesting results.
For instance the Winds of Magic adventure suggests that channelling rolls add one Expertise die and one Challenge die. Higher risk with chance of extraordinary succes.
How often do you use this option in your campaigns if at all?
Cheers
-L