So DMARC is just advertising whether you think your SPF and DKIM are set up correctly? Seems useless to me. SPF already specifies what to do with messages that

So DMARC is just advertising whether you think your SPF and DKIM are set up correctly? Seems useless to me. SPF already specifies what to do with messages that fail SPF. SPF is necessary. DKIM is questionable. DMARC is useless.