We are applying Rule 2 for a reason - when handling a firearm you don't casually muzzle the crowd around you and endanger them. This could be a range, gun show, sporting goods counter, or your friends house where you are over for dinner.
Of course we apply Rule 2 then - it's not only safer, it's also part of gun culture and is becoming extremely ingrained as courtesy. The armed public should be so concerned about knife etiquette - the rare time I loan a knife I get it back in some other condition that what I handed it.
That's not what is happening in a defensive gun use situation. At all. You first heard or saw something out of the ordinary and once processed you are responding to a LETHAL THREAT - or so you think. YOU ARMED YOURSELF in response to that threat and are trying to make a determination if it truly is lethal. So far so good - we are exercising due diligence instead of allowing fear to cause us to overreact.
If someone is coming thru the bedroom door at the point you have a gun with light on it, you have a very short window of opportunity to make a shoot or don't shoot decision. You light them up anyway - it's not a well lit gun store downtown, it's your home with something threatening going on.
Family? Ok, the weapon was on safe, right? And you both reach a new level of awareness.
Intruder? Your choice, but the next move may need to be off safe and shoot. If your gun was pointing at the floor you will be slower - they may still be moving and now even more quickly as they spot you up and out of bed.
There is a new perspective that a light in CQB use may not have to flood the room destroying your night vision. It can illuminate the target with a small spot beam which is also the "sighting" mechanism. A laser diode driven light with a 12-18 spot at ten feet in your dark house at night can illuminate the features of a human enough to recognize who they are - and also give you a COM shot if you need it.
Nothing wrong with the intent of Rule 2 for general use when handling firearms but forcing yourself to apply it when in a lethal threat situation can endanger you more. If you think it's a lethal threat - light them up and decide. You may not have enough time to do anything other than pull the trigger.