Should all e-book formats have ISBNS?

On the Publishing Perspectives website, Erik Christopher asks whether publishers assign a unique ISBN for each e-book format of each title they publish.

[While] there is no universal answer as to whether or not each format requires individual ISBNs, one thing is indeed clear: take control of the process. The worse thing you can do is lose control of your content or let another entity (whether conversion house, distributor or retailer) control the metadata and, accordingly, the invisible ties that bind you to your customers. Ceding too much control takes you out of the picture and makes this already complex situation all the more challenging.