Why PowerPivot requires "classic-mode" web applications
SharePoint 2010 has a new ‘claims-based’ authentication system that allows you to use federated identities with SharePoint. And there are certainly some customers that are excited to start playing around with this capability. That is neat and cool and all . . . but that isn’t the major reason why folks should be getting excited about ‘claims’. It is just a side-effect. The real reason why SharePoint uses claims is to bypass the Kerberos requirement within the farm. This is a huge benefit for SharePoint. In SharePoint 2007 once you grow beyond a single machine, then you must configure Kerberos between all of the servers – this quickly becomes a huge problem and limits both SharePoint adoption and its growth because many customers do not have the infrastructure needed to run Kerberos. But before you get too carried away with claims, particularly with PowerPivot, you have to remember two things: (a) PowerPivot only support Windows users and (b) we don’t support claims integration down to the client. The first one is easy to understand as SSAS only supports Windows credentials – and Excel Services uses that to establish the Windows identity for the connection. Having claims down to the client means that the user can perform one login to SharePoint – and have it be remembered for all future logins.