I called Apple and was told (basically): Brother provides Apple the drivers, and Apple passes those drivers on via Software Update.

If you find the UPDATED troubled drivers on Brother's site, you will see that they are now version: "Version 4.1.0"

With the following note:

"Corrects the issue of being unable to do 2-sided (duplex) print. (Only for machines supporting 2-sided (duplex) print)"

Brother needs to get this updated driver pushed to Apple ASAP. Not everyone is tech savvy and rely on Software Update from not breaking things.

I'm giving low ratings because of Brother's slow response time pushing this out to Apple. Apple posted the drivers on 12/13, Brother had updated drivers on 12/20.