"but what if it isn't sunny?"
Aren't the lenses interchangeable? You could then just switch them for clear/tinted/high-contrast lenses instead...
Must admit, it makes a hell of a lot of sense. No annoying wire to flap around or get tangled, no unit to attach to your body, etc...
Still at $500 I'll be sticking to my £19.99 non-MP3 quality bargain dhb triple lens sunglasses from Wiggle...