For 5 or less tabs it requires two tabs to browse between tabs, vs. 1 tap for a bottom tab bar that is persistent.
Also as mentioned in the talk the user has just arrived from a launcher, which is why it's poor design to show another one.
I thought you were referring to the launcher style in general as "bad design," not in the context of an in-app sub-launcher which is why it made no sense to me.
As a result, we've seen apps like rdio using the launcher in a way that goes against the user's expectations of how a launcher should work. Instead of zooming the controller in from the center of the screen, the controller gets pushed onto a navigation stack. In rdio's case, they should have built a simple table view with each 'sub-application' having its own row. Tapping one of those rows would then push the controller onto the navigation stack in the same way that it currently does with the launcher.
-- From a holistic user experience applications "spring forth" from the system launcher, creating a consistent feeling of two functional modes in the operating system. The first being finding and launching applications, the second being using the launched application. Introducing a third layer of launched sub-applications complicates that model, oftentimes unnecessarily.
I find that hard to believe considering that launcher concept is implemented by Apple in the iOS home screen.