According to a post on the Lord of the Rings Online forum, Turbine has had to add test server realms to match the demand of players wanting to check out the Helm's Deep expansion content and assist the team with bug squashing, etc.
It already has in some respects. It's the first expansion that we've had since Shadows Of Angmar that required us to add servers to the beta world to match the number of players trying to log in and participate. It was the first beta since Shadows of Angmar that actually tripped our login queues on the beta world and forced us to raise the max player population.
Helm's Deep will launch on November 18th. Check out the rest of the thread on the Lord of the Rings Online forum.