In my book a weapon mounted light is mandatory for
ANY defensive firearm... handgun, rifle, or shotgun.
1. It's dark out 50% of the time.
2. Bad guys love the night.
3. The ability to ID a target as friend / foe is paramount.
Here is the nightmare scenario I use as an example to get my point across...
It's 3:37am Sat morning. You hear a noise. You wake up, somewhat still groggy. There are 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. A man 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, telling him to get out of your house, the police are on the way... Your heat stops as you realize it's your 22 year old son, home from college for a surprise visit... and he was trying to hook up his Playstation without waking you up...
Had you no light, had you been scared, (and you probably will be) had you fired at a dark figure, THINKING he was a thief, and a possible danger to yourself, and your wife... this could have been the tragedy of your life.
Had this been an actual dangerous situation. (NOT you son) you could quickly, and easily come up to the target from that low ready in a fraction of a second to protect yourself, and your family, again, lighting the subject, to get a good silhouette of the sights, to asses the need to shoot, or not shoot, and possibly blind or disorient the intruder.
Yes, you also need a hand held light, for those times when you need light, without a weapon.
You need both.
My daily carry gun has a mounted light...
My defensive long guns have lights.
I also I carry the aforementioned hand held light in my pocket. There will be times (many more times) you need light, but not a weapon, and there may be a time your weapon mounted light fails.
Here is one such thread on the subject.
https://www.thehighroad.org/index.php?threads/laser-light-for-hd-pistol.879729/page-3#post-11758979