> why not just have (snip) one LED that's turned on/off?

I used to have a camera with a battery charger that only had one LED that was turned on/off. The problem with that particular solution to the red/green charge indicator is that you don't necessarily know if it's off because it's fully charged, or off because it's not plugged in correctly or there's no power, and I got burned a few times this way. So to have two separate labelled LEDs would be a nicer solution, one for power and one for whether or not it was charging.