Many of the business rules I've seen would be (and often are) better handled by rule engines; and these tend to be functional or functional reactive in nature.
There's usually lots of complicated conditions; if the customer is grandfathered in, then apply rules R1, if they got discount X, apply R2X, otherwise apply R2. Then a new set of rules come in and you need to grandfather the old ones in, and so on; the conditions grow like weeds. When you have rule engines designed around decomposing these conditions, they make them easier to develop, test, they can do things like warn you when certain cases are impossible to reach, etc.
There's usually lots of complicated conditions; if the customer is grandfathered in, then apply rules R1, if they got discount X, apply R2X, otherwise apply R2. Then a new set of rules come in and you need to grandfather the old ones in, and so on; the conditions grow like weeds. When you have rule engines designed around decomposing these conditions, they make them easier to develop, test, they can do things like warn you when certain cases are impossible to reach, etc.