I have a 2011 Macbook Air and I find that the ambient light sensor does a great job of setting an appropriate backlight brightness level. It's particularly good at not blinding you when you open up the machine in a dark room.
However what I noticed is that it tends to set the keyboard backlight in much the same way as the screen backlight, that is to say it turns it up when ambient light levels are high.
The problem with this is that when ambient light levels are high the correct thing to do is to turn the keyboard backlight OFF because the key markings are clearly visible in this situation. I'd be fine with it coming on if I am plugged into AC power (for optimal visibility?) but this is clearly a waste of power otherwise.
Is there a way to set this? An ideal way to control this is a setting that allows me to configure the ambient light level point at which the machine will switch on the keyboard backlight. So as ambient light level darkens we start with zero backlight, then it goes to a high level of backlight at some threshold level, then goes down from there.