You missed the point of my post entirely. Or maybe you missed the quote of myself after the quote of you. Anyway that's not what I meant at all. I think you took the term playability in gameplay terms rather than performance which is what I was talking about.
I feel quite the opposite. I feel making it dark enough that torches,nighteye,and light spells more than just clutter would not detract from the gameplay at all or make it unplayable.
It doesnt have to be done with real light sources per se because they cause performance loss due to the amount of shadows and particle effects (see real lights for oblivion which replaces all ambient glow with real lights). They can still just lower the ambient light values without causing the performance hit. In the end the only difference is performance (oh and I guess immersion but meh IMO)
Ah, yes, I missed one of the conversations somewhere.
For that idea, it is reasonable to cater to both crowds. People who cannot play the game due to performance should have a slider to turn the game's requirements down, and if people have awesome rigs to make it look good then there is no point in not letting them slide a setting to the max to get the detail they want. I don't see a reason to stick with fake lighting, without options, if it's a case of system performance.
Though, I do take exception to your fleeting view of immersion. It's hard to play the role you want to when you are not immersed in a game. In fact, I'd say immersion is the only thing keeping you in your role and making it more believable. Immersion for the sake of immersion is bad, but this kind of immersion is supposed to add choices and consequences to your actions. You might fear what you cannot see, so will you sacrifice a shield slot for a torch or go back to town for a light spell? You might be sneaking, is there enough shadow in that lit area to sneak around it? etc.
I see true lighting, performance aside, as a reasonable immersion technique because it doesn't matter if you don't want it to. Sunlight, torches, humongous light spells, etc. remedy the issue if you cannot stand it, but it otherwise affects the choices you make from a psychological standpoint.