VRDs sound great in theory, but the more you learn about them the more you realize that they aren't the right solution for the problem. Compared to other AR optics approaches, they're vastly more expensive, more dangerous, and have more limitations to overcome. For example, a VRD will only display if your eyes are looking in one direction (see the North glasses), so a VRD-based headset would need eye tracking (and vastly more complex optics to move the image around) for basis use.

Diffractive optics can achieve much better effects at a fraction of the cost and without any risk.