I don't think it is necessary. I imagine the exploring warbands to light old torches on the walls as they explore the tunnels (both to make them feel better and to have an indication of the back route). So in the beginning with only few tiles explored, there is already lots of unknown dark space, but as they move along and new tiles are placed, they will also light those rooms permanently.
Also, in almost all games we played there were warbands shooting through the many floors, hitting far away powder kegs. Forbidding warbands to use missile fire here changes the balance a lot. Especially the pursuers sometimes need to shoot at the exploring warband to catch up with them (despite the fact that they may double their movement) and being able to shoot at far away powder kegs (which you specifically positioned somewhere) is a very important (and fun!) strategy. So I don't see much need for darkness rules here. I do like the idea of using darkness rules but I find them more neccessary and useful when playing on an "open" board, where models can move in any direction. In these narrow tunnels (and with the "fluff" hinted at above) I don't think this is the right scenario for such rules. But if you enjoy the game more with these rules, then do it! I'd like to hear some analyses of how the games play with such restricted fields of view and if you don't find it too restrictive/boring(?).