Honestly, I think the only real benefit to customized LED notifications are that you can tell the message you get just from the LED without having to unlock the screen. Its pretty minor, but something you get used to. And its kinda cool getting different colors for different messages. . I can obviously live without it, but I had hopes since it seems HTC phones do seem to have notification LED's.
So dev's don't have access to this control in Mango? (BlackBerry requires an App for this too.)