If the file on the server let authenticated users access the file or folder then the users from the other forests would be able to access it. If you had selective authentication would prevent that. Users that need access will need the "allow to authenticate" permission
http://technet.microsoft.com/en-us/library/cc738653(WS.10).aspxYou are correct though that NTFS and Share permissions would still always be evaluated.
Think of something like a print server where many times authenticated users are able to access every resource/printer. It comes into play there a lot more than files and folders which are usually ACLd based on groups.
Thanks
Mike