Let's talk about lights.
There's an ongoing debate in the defensive firearms community about whether one should carry a handheld flashlight, just use a weapon-mounted light (WML), or maybe even both. I want to take a look at this question from a risk management perspective.
In risk management analysis, one of the key questions in assessing the value of a given strategy is "What problem does this help me solve?" In the world of applied violence, there are three different problem sets in which one may want a flashlight.
The first is by far the most common: it's dark and you need flashlight to do some mundane task, no gun required. It doesn't make sense to draw your gun to use the WML in this case--for obvious reasons, a handheld makes the most sense here.
The second is the least common: it's dark and you're hunting known bad guys. This is a common situation for law enforcement, military, armed security, and anyone else whose job tasks them with confronting known threats in low light environments. Here, it makes perfect sense to have a WML, as you're expecting to engage a known threat and the light serves primarily to illuminate the target; a handheld flashlight in the weak hand reduces your ability to engage the threat accurately by forcing you to shoot Strong Hand Only (though there's also the strong argument to be made that a WML lets the known threat see where you are as well, while a handheld held away from the body can help you avoid getting shot yourself). But this is a near zero probability event for a civilian self defense problem--civilians should not be hunting known bad guys in the dark. The only times you're likely to be clearing a low light environment as a civilian self defender will nearly universally fall into a third problem set, the "potential threat" environment.
The third, as mentioned, can be considered the "potential threat" environment: it's dark and you may or may not have to apply violence to protect yourself, but there is no certain threat. In this case you need a light to search for and positively identify those potential threats versus non-threats. Maybe you're investigating a bump in the night. Maybe you're simply walking to your car in a dimly lit parking lot. Given that there is a much higher likelihood whoever or whatever you encounter is completely innocent rather than a genuine threat (e.g., a fellow member of your household, a pet, a fallen tree in the yard, a panhandler begging for loose change, etc) it makes far more sense to use a handheld light in this situation. If in public, the gun should always remain in the holster until you're certain it's needed. If in your own home, it should remain at a low ready position until you positively identify a threat and make the conscious decision to engage. It is critical to identify friend or foe before aiming the gun at the target: Claude "The Tactical Professor" Werner has collected several true stories about people investigating a noise in their home only to unintentionally shoot their own family members because they muzzled them when expecting a threat and fired on instinct without proper positive identification. Unless you want to be responsible for killing your kid sneaking home after curfew, or your roommate when he was just trying to get a midnight snack, avoid anything that requires aiming your gun in the direction you're looking while seeking out potential threats. Avoiding this tragic negative outcome is well worth the disadvantage of potentially having to fire the gun one-handed, and a little dedicated time at the range can greatly decrease that disadvantage. A WML *can* be used to identify targets without muzzling them, but learning how to do so effectively requires training most people are never going to attend, due to time, expense, or interest. Thus, for most people, in most "potential threat" situations, a handheld flashlight makes the most sense.
So if we're asking "What problem does this help me solve," a handheld flashlight is the clear winner for most people most of the time--it solves the vast majority of potential problem sets with much lower risk of a negative outcome than a weapon-mounted light. A WML is a good choice for only one of the three situations in which you'll likely need a flashlight (and even then there's a decent case to be made for a handheld to avoid showing the bad guy where to shoot), and it happens to be the one you're (by far) least likely to face as a civilian self defender. Even if you do have a WML, it just makes sense to carry a handheld flashlight as well.