This is very problematic if user had created a previous ranking (either from other project mgmt tools or other prioritization reports) and pushed to a custom field and want to use that order in a new prioritization reports as a starting point.
If the ability to sort records in prioritization reports with other fields didn’t exist, then users would have to start manually ranking again on new prioritization reports. The report only allow sorting by Product Value and its metrics, and this is a big functionality gap.