Khisanth the Ancient
Explorer
There are still advantages to having actual bright light, e.g. at your chokepoints and kill zones, so the guards can perfectly see anyone crossing the kill zone even if they are out of darkvision range themselves.
On the other hand, a darkvision-having species building an underground structure might want to design their entrances so that there are no sight lines longer than 60 feet (drow would try to make the sight lines longer than 60 but less than 120 feet) and then make it in total darkness. Then, intruders from the surface without darkvision are going to be either totally blind or light themselves up as targets.
It depends on how much they expect to deal with humans (or halflings) though.
EDIT:
I agree that this makes a lot of sense. In complete darkness, all Perception rolls for creatures with darkvision are made at disadvantage. Intelligent creatures would therefore tend to favor light sources. At the same time, there are often large stretches of areas that are completely unlit. In the LMoP Cragmaw Hideout, for instance, there are a couple of rooms with blazing bonfires, but the rest of the complex is completely unlit. The same is true for Wave Echo Cave. This is not uncommon for what I would consider a traditional dungeon crawl.
Yeah, there's no reason for dungeon portions that are inhabited by creatures that don't have access to light sources (or aren't intelligent) to be lit.
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