My personal stance on a plate in the windows is it's even worse than no plate at all.
1-If a cop sees a plate in your window its a dead giveaway it's in the wrong spot. If he see's a car with no plate, the chances of him turning to look at the back of your car (especially if he's focused on traffic looking for speeders or other violations) is much less. Trying to appease the kind of cop who wants to give a plate ticket isn't going to help having it anywhere but where it's supposed to be (esp people who cut down, bend, or wedge the plate in a weird spot like behind the grille).
2-If you crash, that plate (and anything else not bolted down in general) becomes a very heavy, very sharp death machine. I always avoid anything loose, and if I do have anything I put it on the floor behind the seats.