It would seem to me that many large enterprises already have robust directory services such as Active Directory and it would be silly to constantly duplicate users in an application-specific store.
Even if you require duplicating the user store, you can provide a mechanism to authenticate against Active Directory. Alternatively, you could support a standards-based SSO mechanism that leverages SAML.
Support for the XACML protocol. Duplicating information on roles and entitlements is equally insidious.
Support for the SPML protocol. Many enterprise leverage identity management toolkits and would at least like out of the box integration in terms of centralized management and provisioning.
So, why aren't open source projects considering this type of functionality as a default to getting on the radar within an enterprise context?