Actually merchants aren't permitted to ask for additional ID even if you write that on the signature area. They're required to ask you to sign the card properly (or sign the receipt with "See ID" or whatever). Visa and MasterCard have repeatedly told people not to do this.
Oddly enough t was Mastercard who recommended me to do this some years ago after my card number was used fraudulently.
My view is that a signature does not verify my identity. Anyone can copy a signature, but who is going to want to copy your face? In reality a PIN is not much more secure than a signature.
Was it actually MasterCard, or a low level customer service peon at your bank? Because I can't imagine MasterCard telling you to do something that puts every merchant who does business with you in violation of their merchant agreement (trust me - it's in there. Card says "See ID" or doesn't have signature, then no sale).