Well because I wanted to make a FBM out of it to only have to turn one dial.
Yes, I think you can slave the lower jaw rotation to the head morph, but then again I don't know how to do that. ;-)

And I think I would have had stretching problems where the inner mouth attaches to the lips if the rotation didn't correspond exactly to the morph.

But of course I'm always open to suggestions for better ways to do things. :-)
And I have my Lightwave face rig that I set up that can pretty much do anything, which can be used to export any altered parts for morph targets. So I don't have to morph the jaws at the same time.