I waffled on this for a while. I had the regular case (no light) for my K2, and it did a fine job of protecting the Kindle, and when I wanted to read at night I wore my headlamp, which was a little uncomfortable where the strap came between my head and the pillow, but not too bad.
Then I got the K3 and decided to spring for the cover with the light, and I like it very much. I use it sometimes even in lit rooms, if I feel like the ambient light isn't enough, and I read in bed all the time with it. And I take the Kindle places I wouldn't ordinarily think to take a headlamp, so I end up being able to read, for example, when I'm a passenger in the car at night.
I'm not sure I can advise you which way to go, as things like "do you have a good headlamp you don't mind wearing in bed" and "do you often wind up in situations where you'd like to read but it's too dark" are pretty variable between one person and the next. But there are my thoughts on how I got along without the light and why I like having it now.