This isn't "The Director from L4D", this is the human touch of direct influence of game balance globally without the need to release a patch or even a hotfix, and way beyond just how many bots drop at a time. If you want to compare it to something, it might be The Wizard from Oz, pulling the cranks to drive the facade.
If The Director wasn't dumping enough zombies in general, Valve would have to patch The Director to make it do so because it's restricted to its coding limitations. A human Game Master can run it only limited to the variables the devs give him. Open new planets for plundering, change the weather, accuracy of your calldowns, the options are as limited as imagination and time.
Saying this, I don't know why they would leave this all up to Joel, you'd think there'd be a team of 3 to bounce ideas off or something.
Poggervania