Nice summary of the value of business rules in mortgage in Mortgage Technology by Scott Kersnar - Rules-Based Systems Call the Plays. Lots of good stuff in there about why to use business rules in Mortgage processes.
Only thing I might add is to think about how predictive analytics might fit in this - think about all the risk-based analysis that goes on in mortgages. Secondary market risk, credit risk etc. Predictive models give you a powerful tool for integrating this kind of risk-based decisioning into your rules-based decisioning for even better results.