"not only would it be unscary, but the framerates would drop to zero,"
Gotta disagree here. I haven't even finished the game, but I've seen PLENTY of rooms that did not require the flashlight to see, and the framerate didn't drop at all.
I think it's pretty obvious the flashlight mechanic was done solely for theme, not for lack of rendering power. Come on...at the expense of sounding like an iD fanboy...does anyone seriously think that the D3 engine is a piece of crap? Sure, it requires a lot of horsepower to run it...but saying that they went with the flashlight thing to save frames is stretching it a bit.