Many of us have so many subscriptions that we've lost track of them all. Companies also increasingly have subscriptions for services such as software and cloud computing. As customers, they're the source of a lot of value to providers. Chicago Booth's Amir Sufi says this "customer capital" accounts for a lot of the differences in how companies are valued between industries. And it might even have overturned what has been a cardinal rule of business: that revenue is more important than acquiring and maintaining customers.