On top of bedrock in the nether means a significantly reduce mob population. Mobs do not spawn on top side of bedrock in the nether. This allows for all types of mob farm manipulation as well. An even more efficient version of gold farms could be made. Wither skeleton farms become significantly easier to make. There are probably other applications that i cant think of right now.
If the ceiling bedrock is changed to Netherack, i take back my in game rant. That change alone pretty much removes most of the incentive of building in the nether to begin with. In theory that area should be able to spawn mobs now.