> what is wrong with hollowing out the voxel?

from "hear-say":
"If you have only one layer on the outside, it may cause trouble for sloped surfaces. Sometimes gaps or lines appear.The solution is to have at least 2 rows."

-I've personally seen that happening with some of the Blitz voxels.

On the other hand, I presume that game play will be smoother, if the game-engine doesn't have to tow around redundant voxels...