It is best not to assume that your spouse will have total control of your finances if you become incapacitated. Although your spouse has some rights over property you own together, like joint bank accounts, he or she is restricted from doing certain things with that property. For example, generally both spouses must consent in writing in order to sell jointly-owned property or access each others retirement and banking accounts held individually. Because an incapacitated spouse cannot consent, any such transaction would be delayed or prevented.
A power of attorney that gives rights to a spouse often makes transactions easier and more cost-effective.