My unsolicited opinion and some random thoughts....
The rules of engagement are different for cops and civilians. Cops can point guns at people in public that would get you arrested if you were doing that as a civilian. People need to understand that. The badge buys you fair amount of forgiveness that is just not there for non sworn personnel. So with that in mind......
A handheld is a pretty much mandatory piece of equipment whether you are carrying a gun or not. I can walk down any street in the world with a hand held light palmed and not break any laws, morals or ethics. I cannot do the same with a WML.
I can use a hand held to look for something I have dropped in the dark in public. I cannot do the same with a WML.
I can use the hand held to get someone's attention in public in a crowded venue by simply flashing the light and drawing their attention to me. I cannot do the same with a WML.
I can use a handheld to navigate my way through any building in the world if the power goes out. I cannot do the same with a WML.
I can use the already palmed light as a distraction device/OODA disruptor as the opening to a physical response (preemptive strike or clinch) to failed MUC. I cannot do the same with a WML.
I can use the handheld as "hammerfist helper" impact weapon if it is already in hand while managing the unknown contact. I cannot do the same with a WML.
I can use the light as a distraction device/OODA disruptor to help buy me a tiny sliver of time to draw my pistol. I cannot do the same with a WML.
I can point a handheld light at ANYONE I choose to in public and at worst I will merely annoy them .... whereas if I point my WML (and the muzzle above it) at someone who I cannot clearly articulate needing to be shot I have arguably , by definition, committed aggravated assault.
All of these things (and many more) tell me I need a handheld out in public far more than I need a WML.
So once we have established that the handheld is probably a far better "plan A" in public for all the pre fight stuff then where is the WML useful?
Once the pistol comes out I am going to have to either shoot 1 handed or adapt my 2 handed grip to accommodate the hand held light. I do not have to do that with a WML.
Once I have established that someone needs to be shot the WML makes further illumination of them and their general area simpler.
Searching through a structure (which is generally a horrible idea for civilians) for unknowns (or known antagonists/combatants) is easier accomplished with a WML (yet it is still doable with a hand held ).
Using any long gun with a WML is easier than using any long gun with a hand held. You can still use a long gun with a hand held but it is much simpler with a WML.
Bunkering up in the bedroom, safe room, panic room, closet, ( or whatever) and waiting for them to come into the field of fire of your long gun is easier with a WML. At the point that people are illegally inside your domicile and coming to you , it can be argued that you still have an ethical and moral (if not legal ) responsibility to positively ID them before filling them full of buckshot or a half a mag of .223. A WML makes this have less moving parts than a hand held does (BUT this can also be accomplished with a strategically placed hand held left turned on....story for another time )
A WML (on a handgun) also serves as a spare light (assuming you have a hand held also) that you can use any way you need to. Want to use it as an "area denial tool" in a darkened structure? Take it off the pistol and turn it on ...now whoever enters the room or hallway is illuminated and you do not have to be located directly behind the light source.
A WML makes checking out the "bump in the night" in your boxer shorts and t shirt a lot easier. With only the gun itself to pick up you now have a free hand to turn door knobs or whatever. It can be done with a hand held but not as easily and you are only having to pick up 1 item since the light is attached to the gun. The home defense role is where the WML really shines (I'm a punny guy) for the civilian defender.
The issue that I (and many others) have with the WML on a CCW gun is simply that the hand held does not point a muzzle at people and can be used BEFORE the gun comes out and the fact that once the WML in use you have now introduced lethal force into the equation. Period. As a police officer that is one thing...as a civilian it is something different. Once the gun comes out in public -especially urban areas- SOMEONE is committing a crime (either them before you drew or you now). A hand held light is far more forgiving in regards to where you point it and frankly far more useful for a wider range of tasks...like convincing unknown contacts that they are barking up the wrong tree. That is not something you can use a WML for without pulling the gun along with it....and trying to KEEP from having to pull the gun is why we avoid , deter and deescalate if possible.
For private citizens WMLs are at their best when attached to their tools they use to repel boarders (home defense guns). John Correia (Active Self Protection) has now watched and cataloged over 17000 gunfights on video and according to him exactly ONE would have been effected positively by having a WML on the defenders pistol...and that was a home invasion break-in in a darkened structure.....not a street mugging.
That does not mean the WML is a BAD idea on a CCW pistol ? No. No it absolutely does not mean that. It just means you will STILL have to use the handheld for 99+% of your illumination needs and you will (if you are like most people) need to fight that urge to "gets some use out of the expensive accessory" attached to your pistol. Unless a crime was being committed (or you can articulate why you reasonably believed one was about to be) your gun does not need to come out of the holster...as such your WML is a moot point unless a crime is being committed.... and as a non sworn civilian you are looking at a TINY sliver of incidences where the light on your gun will have any useful effect.
Me personally? After much training and practice and thoroughly looking at all sides of the equation I have WMLs on the home defense guns and no WMLs on the EDC guns. But that does not mean I NEVER will. And that does not mean I'm the final answer in YOUR particular situation. Do your own research and make your own informed decisions.