you can...but if you stack in to befoul you loose alot of damage potential. and resto staff beats it every time.
useful in a group i guess.
You have put 22 points in Befoul, increasing Major Defile's debuff by 20%. The size of that debuff is 30%. You've increased that 30% value by 20%.
20% of 30 is 6. So you've increased Major Defile by 20%, from a value of 30 to a value of 36.