It's an interesting pitch - but as a customer of say, SendGrid, why wouldn't I just continue to use them?

Would this be for organizations who send so much email that SendGrid (or Mandrill/MailGun/etc.)'s pricing wouldn't be cost-effective?

This works along side your current senders, it doesn't replace them. I take a look at your senders, install some analytics, and recommend DNS changes that boost your deliverability.