Nether Brick slabs added in the most recent snapshot occupy an already existing and well used metadata.
43:6 and 44:6 have been for countless versions known as "Seemless Stone Halfslabs". While not actually craftable these slabs have been available through mods, /give, server economies and WorldEditors.
By giving the 6 damage value for slabs to Nether Brick slabs you're making some maps not future compatible and leaving them in the past, you're destroying existing builds. A build using a brighter seemless slab build will most likely not work if turned so dark.
This issue could easily be solved by giving Nether Brick slabs Metadata 7 instead.
Attached are screenshots of block 43:6 and 44:6 in 1.4.5 and 1.4.6