The main reason people give for not wanting a frontlit reader is that the light diffuser somewhat lowers the screen contrast, by interposing an additional layer between the text and the reader. That's a valid argument, to be sure.
However, by buying a reader with a frontlight and then turning the light off, you're in the worst of both worlds - you still have the light diffuser reducing the contrast, but you no longer have the light to improve it again. It just seems crazy to me
. Either buy a device without a light diffuser, or buy an illuminated reader and use the light for what it's there for!