Only thing I can think of is to adjust for people who can't see clearly at VR focal lengths (~1-2 meters from what I can find, which is a sweet spot for many, but not all). In that case, space around the lens exterior doesn't matter much, it could be a great fit.

Eye tracking and foveal rendering: I really hope we get that, and soon. It'd look more natural, and seems like it could substantially reduce rendering costs.