I thought you're proposing to drop back-facing sides of all bounding boxes which are 'invisible' from the point of the screen plane (where you shoot your rays from)?
If you meant compressing the size of bounding box structures by re-using parent node coordinates for child nodes, you're probably confusing BVH tree with
binary space partitioning tree.
In 3D space, child bounding boxes might share some vertices with the parent bounding box, but there's no way children could share 6 vertices with parents - it's only possible to have 1, 2, or 4 shared vertices (and even then, straight angles were a thing for first-get
2.5D first-person shooters like
Wolfenstein 3D,
Doom,
Duke Nukem 3D, and
Star Wars: Dark Forces).
Here are some visual examples. First, consider the blue parent bounding box, which has purple and orange child bounding boxes each sharing a single vertex with the parent (so 7 vertices are unique for each child).
View attachment 9447
If you stretch up each child box along a single axis until you entirely fill up one side of the parent box, there would be 2 verices shared with the parent (and 6 unique vertices).
And if you stretch the children along two axes to fill up the entire side of the parent box, they will now share 4 vertices (yet 4 vertices would still remain unique).
Obviously the next step would be to fill the entire parent box until all 8 coordinates are shared, which doesn't make sense.
View attachment 9450
But even one shared vertex would be rare - most child boxes would only touch the sides of the parent box:
So there's too much hassle to account for cases of shared vertices, which would require additional vertex attributes or vertex masks to indicate shared state, incurring addtitional processing overhead for very mild memory/bandwidth savings.