I think the main disadvantage of the public cloud is that your data is out there beyond your firewall.  The main disadvantage of the hybrid model is that it still requires that you maintain a greater portion of your IT infrastructure.  

The hybrid cloud offers a number of benefits.  It allows you to retain sensitive data behind your firewall while taking advantage of the lower cost and flexibility of the cloud.  It can also improve scalability and provisioning at decreased cost, allowing resources to be allocated to the cloud for short term projects at much lower cost than it would to make changes to your infrastructure.  One often overlooked advantage of the hybrid cloud is that it allows companies that are reluctant to move to a cloud computing model the opportunity to "dip their toes" into it at low cost and with less perceived risk.  This can be a significant benefit for those who work in a company with management that takes a very conservative approach to technology.