Police carry a small flashlight with them. You can hold it if needed. It doesn't need to be attached to the rifle, the same way it doesn't need to be attached to a pistol to be used. Plus you want to be very careful with the light. Most people not on the same side don't use lights. You walking in with lights etc makes you an easy target. Even going in with it off then flashing it on to check give you away. While you light up to identify they just shoot your light. When going in a non barricaded area to clear, you go quick and fast. You can also in many cases use other lighting tricks. I'm not going to get into those here, but they have proved more effective and less dangerous than turning on your light. If barricaded and in entry teams then you're going in slow and light etc is fine as you have extra eyes/firepower. Plus generally upgraded armor. Flashbangs, shock/awe etc.
Unless you specialize in urban/residential clearing then 99 percent of the time that light is just dead weight if attached to your rifle. Do you use it in the day if outside? In fields, from outside a house covering the perimeter? At the range? If infantry moving in the day? I'm pretty sure if you were regular infantry and in position that if you turned on that light whether on purpose or by accident that your own guys would want to shoot you. Again if specialized for house/urban clearing then sure. But for regular duty. I'll stick to the hand held light. I can use it with or without anything.
One other point. On the range you aren't aiming that carbine, or pistol at a target for sometimes hours. Having had to have my pistol aimed at a front door for well over an hour with a barricaded suspect I was very thankful for 1) The car side mirror I used as support, 2) not having a lot of weight up front as it's a short pistol. The ERT guy was having to hold his AR unsupported. I noticed he was getting tired. I don't blame him. That stuff adds up. You know it wasn't because the guy was out of shape. Physics dictates the lever effect. The more crud up front the heavier it is. Even more so than the actual weight.
Again for specialized I can see it. But for 99 percent you don't need it. You can always attach one later for specialized use.