I am against beam weapons for regular police officers as I believe they will be abused even more than Tasers. I think they should only be issued to experienced officers with good service records who pass a special training course and pass an annual psych evaluation. I believe that the typical street cop is exposed to so much unpleasantness that it eventually becomes too easy for them to use these devices on people. Also, many entry level police officers barely get enough training as it is without complicating things with the ethics and applications of a plethora of less than lethal weapons. (There are a lot of these "less than lethal" weapons being developed, including flashlights that make you vomit, directional noise cannons, Taser shotguns, etc.)

The more often you are allowed to easily zap other human beings into meek obedience, the less reservation you are likely to feel about doing it. I believe that this (and lack of training) is why we have so many questionable incidents with Tasers being used on people, such as in these examples:
http://www.youtube.com/watch?v=ysDZo_4t9ao
http://www.youtube.com/watch?v=fizo-sOSE6o

If entry level and poorly trained police didn't have torture devices to use as compliance tools, they would have to behave differently. For example, in the above video the police would not have been likely to shoot the unresponsive diabetic because they aren't authorized to casually use lethal force against nonthreatening targets. If they had been forced to grapple with the diabetic instead of tasing him, they would have been more likely to realize that they were dealing with a medical emergency instead of a criminal.

I think the pain beam will be even more tempting to use. It doesn't leave marks or evidence so it will be difficult to prove a beam was ever used on somebody. Think of the fun in using it to disperse a group of loitering thugs and watching them scramble. If some jerk flips you off because you are a cop, why not give him a taste of the beam? If you have a suspect in custody, why not handcuff them to a chair and beam their crotch until they confess rather than worry about 5th Amendment rights? See a member of your least favorite race? Why not give them a zap as you drive by to let them know how you feel? It won't leave a mark and nobody can prove it ever happened. After all, your word as a cop will be believed over that of victims.

Although I am staunchly in favor of everyone (civilians and police) owning self defense weapons, putting non-marking pain compliance devices in the hands of police makes me uncomfortable. If I as a civilian use it for anything other than self defense, it will be considered assault and battery and I will go to jail. If a police officer does it, chances are good that they will get away with it as long as there aren't multiple witnesses.

We pay officers to take risks in order to protect civilians. In my city, a city police officer fresh out the academy makes nearly as much as people at the upper end of my profession after extensive education and a decade of experience. They get this money because we knowingly put them in harm's way. I would rather most of them not have access to torture devices even if access to these devices would slightly improve their safety. Police have been able to keep the peace for hundreds of years without these devices.