this is because the controller is wireless i.e battery powered, the battery's don't supply enough current to be able to properly power them very bright (unless the battery's are brand new, then they will be bright)
you're wrong... if the controller is off, with any kind of battery in it none of the lights should be lit, untill i press the Guide button
but that's in the first post, if you'd read:
"but when i press the guide button, all 4 leds light up as they're suppose to.."
... already been swapping LEDS on Charge Base, Wireless Headset, Play&ChargeKit and my other controller.. i like soldering