This has always been a bur under my saddle. A patrol vehicle should be highly visible for several reasons: deterrence by way of visible presence and quick recognition by the public. In other words, the bad guys need to know the cops are around and the good guys need to be able to recognize a cop if they need one. If there's one thing I've learned in a quarter century of uniformed patrol work it's this: the general public largely moves through life with their heads up their ass. If you want to catch traffic violators you don't need an unmarked car to do it. You can paint your cruiser tangerine orange with lime green door panels and most of them won't see you until it's too late. In twenty seven years I've made exactly one stop in an unmarked car. I was in plain clothes and driving an unmarked car at the time, while going home from an assignment. The only reason I made the stop was due to the hazardous nature of how the vehicle was being driven. It was one of of those, "stop it now before it kills someone" situations. As soon as I was able I radioed for a marked unit and handed it over to them. The whole process made me very uncomfortable and I felt strangely exposed the whole time.
Officers themselves like unmarked cars and low profile graphics because they think they look cool and tough. They may give you some kind of bull shit tactical excuse, but that's really the underlying answer if we're being honest. They like cruisers that look tough and blacked out because they fall victim to the cool kid syndrome just like everyone else.