I suppose that any tool can be used in a beneficial manner assuming the user trains with it extensively. I have seen Crimson Trace laser grips on guns in three different classes and by and large what I learned was that the shooters tended to be a little more accurate and a whole lot slower. This was for indoor and lower light shooting. The difference in time was slight by the clock, but a whole lot of time when considering a gunfight scenario. With lasers, the students ran 0.2-0.3 seconds slower. In double tap drills, I would be finishing as the person with laser grips were firing their first shot. When I have seen them used in mid day sun light, they were often hard to spot unless you knew where they were supposed to be already. Shooters tend to spend a bunch of time trying to find the hard to see dot. In such cases, shots were often a full second or more slower.
I always carry a flashlight, usually a Scorpion, but I also have a Surefire 9Z as well. None are weapon mounted. With any gadget, there is always the chance of it not working, but with a gun mounted illumination device such as a laser or light, the one malfunction that I am fearful of is the one where the laser or light don't turn off and so the opposition will have an idea where you are at all times. If it happens with a handheld light, you can just toss it away.
The only weapon mounted light I have considered is the replacement pump stock with the Surefire light built in. This would be for my home defense Rem 870 shotgun. Its use is very specific and the light would likely be advantageous in that specific use of clearing my own house. I would like to keep both hands on the weapon and this would be hard without a weapon mounted light.
With whatever accessories you choose, remember that until you are extremely proficient, they may hinder your abilities more than help them.