The Tension between Public and Private Clouds

Last week, during discussion on cloud interoperability and standards in Israel, I saw for the first time a real dichotomy in the value of public (external) and private (internal) clouds. This tension seemed to arise from the fact that an CIOs considering moving applications to an outside cloud vendor, would probably set their highest priority on legacy applications. The logic was that since it was more costly to maintain older applications internally, moving those applications to the cloud would represent a high value option. This customer high value option, however, seemed to present a worst case success scenario for public cloud providers. Is this true?

The general lack of internal metering for applications would also make an Internal vs. External ROI business case a fairly difficult task. Could these tensions actually lead to different business models for internal and external cloud purveyors?