There's no reason to hollow a solid block of geometry unless you want players to access it or put details in it behind some glass/chicken wire window, so no; but for the sake of tall structures, it's often the case that textures tile vertically with dirt gradients at the base/top of the texture. So it's sometimes necassery to cut a block into 2 if they are taller than 256 units. But there's still no reason to hollow it even then; parallel faces of solid geometry against other solid geometry get culled anyway so if your 2 stacked brushes have a non-nodraw texture on the faces that touch it doesn't matter.
