[MENTION=6778044]Ilbranteloth[/MENTION]
You seem to be agreeing with me for the most part, with some parts at odds. So, forgive me, but it does feel like you only read the start and end of my post rather than the whole thing...
I read the entire post. Although you mention that "illustrations show drow cities using light" you also have a thrust that says that we (humans) make assumptions that they would use light in their cities or elsewhere, and that it's a humanocentric approach, such as indicating drow house devices would be about shape and not color, for example. So the thrust of the entire post, despite acknowledging that illustrations show the use of light, is that they would use it as little as possible if at all. While you seem to be focused most on combat situations, that's the feel I got from your post. In addition, my thoughts are for the benefit of the larger discussion, about whether underdark races would use light, and whether darkvision is really such an advantage.
Since a lit object in an otherwise dark space is so visible, I suspect that intelligent underdark would use light liberally to their advantage. Drow have an innate faerie fire and dancing lights, both of which are strategic spells in the dark for this very purpose.
The larger discussion, to which I was addressing, is focusing primarily on combat advantages, which is what a large amount of your post is doing as well. That's all well and good, since players often look at D&D through the lens of "what makes my character better in combat." My position is to start from the opposite direction - what would a civilized culture do? The use of light in human development was probably a secondary development. Fire was most likely most important for heat first, then perhaps for light and/or cooking. It was also useful for safety as most animals are afraid of it, which probably applies to animals of the underdark as well.
In addition, drow lore in D&D is that they started as a surface race. So they may have embraced the darkness over time, but they came from a culture that had fire, light, and magic. Their use of light strategically would certainly be different than a surface elf at this point, but would still be something they were already familiar with. Their use of light would also differ from that of a race that evolved entirely in the underdark.
The most common threat in the underdark are probably the equivalent of predator animals/monsters on the surface world. So drow, dwarves, and other intelligent underdark races would likely take advantage of the use of light to keep predators at bay, as those creatures most likely prefer to remain in the darkness.
Also, while I consider derro and duergar as Underdark races, dwarves are not. They are subterranean, but not Underdark. Their greatest fears are probably related to the true Underdark races who have better darkvision and are more at home in the dark. Goblins, orcs, and other such creatures are always a threat, but they are a much more known threat, and probably use light as well for similar reasons - to push the true dark as far away as possible. Patrols might very well carry light as well. A patrol is not attempting to be stealthy, they are usually attempting to announce their presence to assert their dominance over a region. The light serves as a warning to those that would trespass. A patrol is hoping not to engage in combat, just observing and driving off threats, hopefully due to superior numbers and strength.
Criminals will avoid the use of light where possible. Of course, an drow rogue has the same chance against a drow rogue as a human rogue does against a human rogue. A human rogue cannot see in total darkness, so they will take advantage of the shadows, of dim light, to give them an advantage. A drow rogue will stay to the darkness, which is essentially the same as dim light to a human. Again, the point is to consider the use of light from the perspective of the drow world.
Military movements will use light or no light depending on the type and purpose of the attack. Stealth attacks wouldn't use light. Large scale attacks would - especially when ranged weapons are in play to extend their usable range. In the Underdark, all creatures generally have darkvision. So your darkvision isn't any more of an advantage than theirs is. Taking that away levels the battlefield. So developing tactics and weapons that take advantage of the use of light can actually give an advantage against those that don't. This would include most monsters and predators.
So I also disagree with your characterization of our consideration of the drow or other underdark races using light as humanocentric. If you can only see 120' before darkness, and there's always a potential threat 130' away, the natural instinct for an intelligent creature is to increase visibility. And that is done with light. In earlier editions, there were spells like
darklight which is light a light spell only for creatures with darkvision. That's an option, and probably a reasonable one, although it's also a spell that
would tilt the advantage significantly against surface dwelling creatures. I personally don't have a problem with that. But I also don't have a problem with the number of races that have darkvision, nor do I have a problem with darkvision races using regular light as well. I think it would be more the norm than the exception. The main exceptions being situations requiring Stealth, although even then I think it would be used as an effective distraction.
