We're all issued a flashlight. Some of you who work days don't always carry it with you, but that's foolish—you never know when you'll have to search a dark cellar, warehouse or garage. With all of the inexpensive, small, high-intensity flashlights on the market these days, there's no reason not to have one on your belt at all times.
Further, you should have a second light handy, for the same reasons: Today, high-output lights are so compact and inexpensive that not having a spare in your gear bag is foolish. Your primary light may fail (all things man-made do, and batteries only last so long), and when it does, it's a sure bet it'll fail when you need it. In fact, consider carrying a larger, higher-output light in your gear bag, where space and weight aren't as important as on your belt. Not only is this larger light redundant, it's extra light power you'll be glad you have when you need it.
There's almost no part of our job we can do if we can't see well. A good light—or better, a pair of good lights remains as essential to our performance as our pen, gun and cuffs.