The problem if games are too dark is that I can't see anything, which is bad when you need to defend yourself. There's a difference between atmospheric shadowing and not being able to see movement and getting lost. I call it the difference between 'living darkness' and 'dead darkness'.
Living darkness acts like actual light and shadows - you can see it move over characters and objects as they move. Fallout 3 and L4D had great living darkness. There are so many great moments where this shadowy figure is rushing towards you, and it's only when they're right in front of your face that the light hits them and you can see every detail on their face. It's atmospheric, and you don't have to, or even want to turn up the brightness in order to play.
Dead darkness just looks like everything in a location has been painted black. There's no ambient lighting within the level, or it's not emitted realistically. There's just intermittent blotches of light and darkness. As much as I love the game, Oblivion did this in its underground levels. I get that it was supposed to create atmosphere, and you were supposed to make a choice between using a torch or sneaking around, but, hell, the game was just so dark at times that I couldn't see anything, and it actually detracted from the experience. I had to turn up the brightness.
It's like those 'gritty' TV shows or movies which have scenes filmed in near-darkness, so much so that you can't see what's going on because there's no light hitting the actor's faces. You can't sacrifice coherence and gameplay for atmosphere. But, fortunately, I think the ability for game designers to animate light and darkness has vastly improved recently, so I can settle down and enjoy my nice dark caves. =D
