Even from a humanistic perspective, why not require either that, or an oral statement UNDER OATH. If it "inconveniences" families who are simply bent on doing a relative to death without his or her being required to show formally that's what he or she wants... GOOD!!!!! The government is SUPPOSED to be God's wrath upon evil.
If you hired me as your attorney, I'd definitely advise you to put your wishes in writing, make numerous copies and give them to as many people who you could.
My advice is free on FR, but I don't give a warranty with it.
Terri's case wouldn't even be a case if she had done that.