Lets Assume A High Quality Weapon Light...
...Like A SureFire X200B.
The advantages of a weapon mounted light are self-evident.
Presumably, the light is always on the gun, where it's needed, dedicated to one purpose.
With a weapon mounted light you have one hand free to open and close doors, dial 911, or let loose the dog!
With a quality weapon mounted light you can flash-light an entire room by skipping the beam off the ceiling--more than enough to see if the room's occupied by someone other than the wife!
In the direct glare, the light will surely momentarily blind the perp, giving you a chance to appraise whether or not they're armed (I know the old argument that you can hold a flashlight well away from your body, so if the intruder shoots at it... Well, who says he's a good shot, and won't hit you anyway? Besides, you can remove the weapon light from the gun and do the same thing...).
Held at gun point, the perp is disadvantaged by the glare, while you can clearly appraise the situation, while still having a free hand, if needed.
Proper use of a weapon light requires training, and/or well thought out drills--just like a laser does! You don't stroll through the house with a point and shoot beacon blazing!
Cops and soldiers alike, consider their weapon lights iindispensable.
I'll attach a pic, but my jpg pics are usually rejected by this site...
--Ray