Saw this post on the EDS blog - Does the rigor of business rules free you up to be innovative? Charlie Bess refers to a piece by Ron Ross called Business Rules: When is a Door Not a Door? Like Charlie I think that control of your rules gives you more flexibility as well as more rigor. Let's face it, most systems are so hard to change that you know they are out of date (and therefore not rigorous). And if you can't prove something is rigorous (because no-one who understands what that means can read the code) then does it even matter if it is?
There's a lot on the blog already about business rules (including an FAQ), business agility and compliance (often the purpose of rigor). Using business rules to make what you are doing explicit, and putting the power to change that in the hands of your business users makes you more (and not less) agile while retaining the rigor you need.