I'll be the first to admit that my technical experience in this field is limited, but I have been following the tech for some time.

Transparent display tech has come a long way since its inception, but from either a transparency standpoint or as a high fidelity interface for VR I can't see how it's a viable option. Given the choice I would much rather have a crystal clear UI with simulated AR than compromise display quality with adjustable transparency.

Why Apple chose to develop the googly-eyes outward-facing display is beyond me though. Maybe it looks better in person than in their promos.