ICD9 Volume 3 codes on outpatient encounters?

I have been a CCS for over 10 years and I have recently encountered coding professionals who insist on coding ICD9 Volume 3 procedure codes on outpatient hospital encounters, in addition to CPT® codes. The official coding guidelines state that Volume 3 codes are approved for inpatient hospital procedures. Did I miss a Coding Clinic somewhere along the line? Why are they so adamant the Volume 3 procedure codes are required on all hospital encounters, including outpatient?
Thanks!

re: ICD9 Volume 3 codes on outpatient encounters?

Some payers want both CPT® & volume 3 codes on the claim for outpatient services. Medicare does not, but some managed care/private payers do. Likely they have encountered a payer that will not complete the processing without volume 3 codes.