Meaningful agency is important. Your spouse may best effectuate your wishes while you are incapacitated. But that's not always the case.
This is the kind of thing that makes me yearn for living wills implanted in subcutaneous microchips. If vets can tell who a dog belongs to with a quick scan, docs should be able to obtain some accurate idea of your preferences as to medical care in similar fashion (the default rule in the absence of chipping being resuscitation).