While the above are all very interesting uses of keyboard lights, they still don't seem to qualify as "something useful". From my point of view, they're right up there with case mod
But fear no more!
Your LEDs can be put to something that actually is useful in certain circumstances.
Which ones, you ask? Let's take the hypothetical situation in which you are stuck in a prison in the Phillipines, with massive amounts of free time, a laptop, and some important information to decrypt. Furthermore, you are quite certain that you're the victim of a massive conspiracy spanning some 50 years, and this conspiracy has the power to find out what's displayed on your monitor using Van Eck Phreaking.
So, pipe the output of your decrypted text to the keyboard lights. Make the lights blink in Morse Code to figure out what the text is. This will take you massive amounts of time, of course, but, after all, you're in prison. Also, you have the kind of ruthless discipline required to sit through the blinking of the LED and figure out what it's saying because of your sudden aversion to masturbation.
OK, I'll be honest. I didn't come up with this. Neal Stephenson did in his Cryptonomicon. But it's the one use of keyboard lights that's not utterly useless.
So, rejoice, conspiracy buffs, and take off that aluminum foil hat! You can now re-read your important JFK assasination theories using your keyboard! Hurrah!