If distance is a primary factor, iron sights can be effective but the range of most rifles will far exceed the ability of a shooter's eye. This is especially true considering the need to identify targets or take a very accurate shot. An optic with the capability of magnification virtually eliminates these limitations and can greatly increase the effective capabilities of that officer. Variable powered optics, when properly paired with a weapon system, can be incredibly valuable tools in the hands of a capable and properly trained patrol officer.
As we stray into the realm of variable power optics, the options are almost endless. The amount of magnification needed, the option of illumination, and the type of reticle are all very important things to consider when looking for variable magnification glass. There is a significant difference between a scope designed for hunting, one made for sniper or designated marksman use, and one designed around a patrol function. My suggestions would be to keep the magnification range to a minimum, keep the reticle simple, and ensure the scope has good illumination.
A few good examples of variable power optics are the Leupold MR/T 1.5–5X, the Bushnell Elite Tactical 1–6.5X, and the Trijicon AccuPoint 1–4X. All of these optics are very durable and designed around a medium- to close-range tactical application. The magnification at their highest setting can assist greatly with target identification and engagement at distance, and at the lowest magnification they can still be used for close- quarter engagements. Essentially, these optics provide the "best of both worlds" and are likely the best option if your needs are varied or your budget only allows one rifle for many applications.
Test and Evaluate
So with so many options out there, how do you choose the right one for your application? Do your homework. Real-world testing, preferably with an example to put through the paces, is the most obvious. Most reputable optics manufacturers will gladly provide a sample for testing, most at no cost.