In my book a weapon mounted light is mandatory for any defensive firearm, handgun, rifle, or shotgun. It's dark out 50% of the time, bad guys love the night, and the ability to ID a target as friend/foe is paramount.
Here is the nightmare scenario... It's 3:37am Sat morning. You hear a noise. You wake up, somewhat still groggy, and there is shuffling sounds coming from the living room, then a bang of something hitting something else, and a muffled voice cursing. (someone is in the house) You grab your light equipped handgun, rifle, whatever... and wake the wife to have her call 911. As you round the corner of the hall to check things out, you see the shadow of a figure of a man who appears to be unhooking the big screen TV in the living room. You bring the weapon up to a low ready, pointing at his feet. (ready to raise and engage if needed), You hit the light for a second, telling him to get out of your house, the police are on the way. It's your 22 year old college son, home for a surprise visit, and he was trying to hook up his Playstation without waking you...
Had you no light, been scared, and fired at a dark figure you thought was a thief, this could be the tragedy of your life.
Had this been a dangerous situation, you could come up to the target from that low ready in a fraction of a second to protect yourself, and your family.
Yes, you also need a hand held light, for those times when you need light, without a weapon. You need both.