Actually, one reason I haven't bought a kindle is because it does not contain a built-in light. (The other reasons would be cost and DRM)
I like to read in bed before I go to sleep, and having to use a desk lamp or a book light is rather annoying. I currently read some ebooks on my iPhone which works pretty well on its lowest brightness setting.
I would prefer a bigger / better resolution display, though, which is why I have atleast considered some ebook readers. But having to turn a light on just to read? Not really an option for me.
Now, during the day an eInk display would be wonderful! But it needs to work in the dark as well, atleast for me.