The obvious disadvantage is, that if you want to illuminate something, you have to point your gun at it, which could cause rule 2 violations.
A good light will put out enough non-axial beam that you can illuminate something with the gun in low ready or high ready; unless you are using a really cheap (dim) light, you don't need to put the core of the beam on something to light it up.
My personal opinion is that you, as a civilian, have NO business searching a darkened building in search of an intruder. Unless you already know where the bad guy is, you should NOT venture out of the room you are in. In situations like that, all the advantages are held by the person who keeps quiet and stays hidden. The moving person will invariably make enough noise to give away their position to the person who is trying to remain unseen. What this means is that you should get your gun, position yourself in an ambush position on the door, and listen. Eventually the person may indeed give away their location, allowing you to maneuver to an advantageous position on them. But going out in search of an unknown intruder in a darkened house or shed will get you shot, almost guaranteed.
You are both correct, and incorrect, IMO.
Yes, if it is likely someone is in the house, you put yourself at a significant disadvantage if you go hunting for them, and it is of great tactical advantage to let them come to you.
However, if it is
not likely someone is in the house, but you want to check out the "bump in the night" for peace of mind's sake, it is prudent to go armed just in case that
wasn't the cat knocking over the vase. Calling your local PD five or ten times a year and asking them to search your house for you because it sounds like your cat knocked over the vase again is not a good idea. If you can have a dog check things out, great; if not, you can check it yourself, or you can lie in the bed and assume it was just the cat.
And, as others have mentioned, those of us with children elsewhere in the house would definitely be moving even if there
were a known threat in the house.
BTW, police are civilians too, strictly speaking; if someone is subject to the civil law instead of the Uniform Code of Military Justice, they're a civilian, whether they have a government job or not.