In Lync Server 2010 the Director is now a dedicated role. Because there’s an actual Director role now you don’t have to worry about users accidentally getting homed on the Director, like you did in OCS 2007 R2. But the real reason to take another look at the Director role in Lync Server 2010 has to do with the new resiliency option. Specifically the ability to associate a backup registrar pool. The Director plays an important role in informing the clients what their primary and backup registrars are.
So the natural question that comes up is “Is a Director required for the backup registrar ability to work?”. The answer to that is technically no, it isn’t required. The Director isn’t providing any special functionality. Just like in OCS 2007 R2, the Front End Servers have the same Registrar service/functionality as the Director. If a user registers against a registrar that isn’t their primary registrar, a 301 Redirect will be returned that includes the user’s primary and backup registrar. A Front End Server or a Director both have the ability to do this. There is another option described below, but it has some limitations of it’s own. Using a Director is the recommended and simplest way to take advantage of this functionality.