Without it, your partner would likely play no role in important medical decisions, and might even be prevented from visiting you in the hospital. A medical emergency card doesn't replace the power of attorney. It is a sensible precaution to carry reduced-size photocopies of the durable power of attorney form in your wallet or car. Only a signed original has the power of law, so be sure the photocopies are signed and notarized. Protect them from damage, e.g. by keeping them in a sealable sandwich baggie.
All states recognize powers of attorney, though laws differ in details.
In some states, you can also use the document to stipulate the type of care you will permit if you become incapacitated. You will need to use a state-specific legal form if you live in California, Georgia, Idaho, Kansas, Nevada, New York, Oregon, Rhode Island, Texas, Vermont, West Virginia or Wisconsin. We can only provide general information, which should not be regarded as legal advice. An attorney also can advise you on other legal precautions, such as wills and relationship agreements.