Good point, Clicker: "Also, a 'fast' lens is a lot more expensive than a slower one and usually has much better optics which may give better sharpness."
When one buys a fast lens, along with better light-gathering capabilities, you're also getting better glass, more finely ground and better coated. Agreed.
But isn't it the glass itself that lets in more light, and wouldn't that let in more light through the range of the f-stops?