The departure of customers after a merger is a top reason why such ventures often fail to bring promised shareholder value, write Bain & Co. executives Laura Miles and Ted Rouse. That's why keeping customers happy has clearly been a priority in the Capital One and ING Direct merger, they write. ING customers were not only promised that their accounts wouldn't change, but also that they would have free access to Capital One's ATMs. "When mergers occur, customers demand consistent and seamless services across both companies from the start. If they don't get them, they defect," Miles and Rouse write.