Lights are really useful against single targets, especially at night or in dark areas since it wrecks their peripheral vision. All they see is the light, and not your squad advancing
But there's a time and a place to use the tac-light, if your enemy already knows where you are, you're not really giving away your position. And they can't see you behind the light, so there's nothing wrong with simply turning your light on and resting it on a crate while you position yourself 5 feet to the left lol