Do I need to import AD users/groups into a database from which StrataFrame can retrieve user data for authentication and authorization?
If you are using Windows Authentication, StrataFrame will authenticate through the Windows API whether you are on a domain or using workgroup permissions. However, you still need a StrataFrame user so that you can assign all of your applications permissions to a user once they login to the application itself. So you may want to write a program that queries LDAP to automatically create SFSUsers records....or you can do this manually as well. We have developers out there who do it both ways.
Or, can I simply use these entities directly (database connection to the AD database directly)?
We never query AD directly. We use Windows APIs for authentication. The issue with not creating SF users is that you cannot assign any permissions, roles, etc. for your application. So even though you are authenticating off of AD, you still need an SF user to which permissions can be assigned.