"All observation must be for or against some view if it is to be of any service." (Source: Scientific American magazine, April 2001)
All performance monitoring must either agree or disagree with some performance model* if it is to be of any use.
* A performance model could be any or all of a SWAG, a back-of-the-envelope calculation, an Excel spreadsheet, a Mathematica notebook, an S-plus script, a PDQ model, a SAS model, a SimPy simulation, a LoadRunner script, etc.
I'll have a lot more to say about this during my CMG Sunday Workshop entitled Moving Beyond Monitoring, Pretty Damn Quick! (December 2, 2007).