Are Accuracy and Repeatability Overrated?

No...

Instrumentation precision is necessary to monitor performance of the system. Yes pumps drift along curves as conditions from sources change, but you can compare performance over time to project back washing or assess maintenance/repair needs. You do not want to have the question arise, with a high level of validity, is it the system performance of instrment measurement error. I had a system where the multiple metered pumps were networked and utilized at flows that varied substantially, and we always had low flow measurement errors at the treatment plant that had to be explained to the Dept of Toxic Substances Control for our TCE pump and treat over a large area of a water basin. We would operate for a few months before we knew that a pump meter was not operating correctly, because the mainline meter always under reported during the non-irrigation season. All our calculations were suspect, since they were tied to concentrations in the inlfuent and flow rates.