The addition of the AR rifle, along with the M&P pistol, shows that S&W may be serious about getting back into being a main supplier to PD's.
The M&P pistol is designed to be the "Glock-killer." S&W listened to some of the complains departments have made about the Glock and designed the pistol to address those issues. One example is that you don't have to pull the trigger to dissassemble the M&P. There have been incidents where police officers have had AD's when field striping their Glocks after pulling the trigger for take down. (Yes, it's a training issue, but administrators love 'hardware solutions' to 'software problems.')
By also offering an AR rifle, S&W can become the "One stop shop" source for PD's. I'm sure thay offer some attractive discounts to departments who place orders for both pistols and rifles.
It will be interesting to see how deeply S&W manages to penetrate the LEO market in the next few years. If they follow the strategy that Glock itself pioneered of taking old weapons in trade and offering the pistols below cost, they might be able to convince departments to switch away from Glock. The availability of the AR rifle is the icing on the cake, as that is something that Glock doesn't offer.
Of course, if the M&P pistol or rifle are dogs, the whole thing won't work.