In the previous games, Underdark was practically an underground desert with no life and filled with random enemies (who most likely fed rocks because nothing else was there).
I really don't know what previous game you're talking about because they were as much life in BG2's underdark than in this one. Big difference was probably that it was less of a place to be...
That said I agree that it shouldn't be completely dark.
It could be a bit darker so torches are more meaningfull but I would be sad not to see the "landscape" at all.
