Other than an indicator of the quality of the lens, I can't see much call for using an f5.6 lens with a 2x TC myself. The resulting f11 lens wouldn't AF on any body (excluding DPAF), and it doesn't leave you many options in terms of apertures for maximum detail due to diffraction.
On that subject, the other day I was wondering if they couldn't incorporate Contrast Detect AF at the same time as using Phase Detect AF in an SLR design? All they would have to do is lay a few stripes of pixels somewhere in either the optical path of the AF sensor or even the viewfinder, and everything would function normally except with the added Contrast Detect AF.
Alternatively they could just put single pixel wide slots in the AF mirror (or both, a few microscopic lines in the viewfinder wouldn't kill its functionality) so that some light that would have gone to the AF sensor just hits the main sensor instead, and they could put colourless microlenses in just those spots to improve light gathering. The only downside to using pixels on the imaging sensor itself is I have noticed more sensor noise when using live view. If a sensor has the capability to activate and read only select lines of pixels (or individual pixels) I guess that would be fine.
If you put hybrid AF in an SLR then a person could seamlessly continue to use AF at any aperture they want (in the case of flipping a TC on during use), it would potentially improve AF accuracy overall, and could even auto correct AF calibration.