Open and click-through rates (CTRs): Knowing who are engaged customers (those who open most emails and end up making purchases) versus inactive customers (who haven’t opened any emails in months) can be invaluable. Marketing campaigns announcing a new product should absolutely include those engaged customers, while re-engagement campaigns can be created to try and entice the inactive customers.
That isn’t to say that sales-y promotions couldn’t ever create results—as marketing “gurus” have proved. The results just aren’t as good as they could be. Take the biggest gurus’ results—the ones they boast about—and calculate their conversion rates. Often their marketing is comparatively ineffective. They just have massive volume, so the sales numbers are impressive.