It appears that this directory is not used at all on RHEL6.
I know I have mentioned this before; but it's true. If I
move my copy of audit.rules from /etc/audit into the subdirectory rules.d and restart audit; the audit.rules file is not recopied/regenerated or whatever by the auditd.
This behavior is different from RHEL7; where if you delete the /etc/audit/audit.rules file or move it to /etc/audit/rules.d/audit.rules; the auditd functions as I expect.