I've never had a backlighted keyboard. But I must admit, sometimes when it's dark I have to look closely at the keyboard to find the key I want, at least to orient myself when I first touch the keyboard.
Not everyone is a touch-typist (although I recently put in the effort to finally learn - highly recommended).
Also, there are some weird keys out of reach from the home row you may need every now and then - the fn keys on a Mac being a good example. Turning volume up/down in very low lighting is a pain if you can't see the keys.
That said, I found the blue LEDs of the Macbook keyboard backlight to be too bright when used in combination with flux on lowest screen brightness, so I wrote a small command line app to reduce it below the minimum normally permitted by the function keys: https://github.com/drhaynes/brightness.c
According to a friend who had cancer as a child, chemotherapy causes you to lose sensitivity in your extremities over time (nerve issues?). I'm guessing the feel of the 'f' and 'j' keys don't help him much.
