- Reproducibility (the official, bundled Excel numerical routines have/had errors greater than floating-point precision) which avoids the unprofessional look of, say, least squares numbers that differ from a check by hand.
- Version and environment control. This is the fastest way I can answer the question, "what would these new routines produce if run against last October's pool of databases?"
- A presentation format where client customizations for style, dimensional units and currencies, human language, etc. can all be owned outside of your project.
I try to sell this approach when I can. Is there a particular BI that strikes a better balance?
As it stands excel is a better presentation layer than almost all BI tools once you're past the modelling and analysis stages.