I have been a PR for 15 years and never gave biometrics. I did give my FBI clearance for PR application and now for citizenship because I've been back and forth between US and Canada these past 15 years.My theory is that one reason is for those who didn't give biometrics before or did it when they are young or for whatever reason (like biometrics under an older system) they don't consider 'correct' anymore.
Only a theory though.