Car and Driver magazine conducted a test to see how sending and receiving texts compared to driver under the influence. Rigging a car with a windshield mounted red light which signaled to the driver to brake, the magazine found at 70 mph: that an alcohol impaired driver traveled four feet further before braking; a driver reading a message traveled 36 feet further; but a driver that was sending a text traveled 70 feet before they saw the light and came to a stop. Compare all of the distractions with a sober driver who reacts in about half a second. Keep in mind that these tests were conducted on a straight section of an abandoned airstrip without any other additional traffic, signal lights or pedestrians.
Sure valuable information can be transmitted to you from dispatchers or other officers in route to a call but the first rule of emergency response driving is arrive alive and the more distractions you deal with, the harder that is to do especially at any kind of speed.
Even low speed driving on city streets or rural back-roads can be perilous as you try to text with both thumbs on your crackberry while steering the car.
To sit in a law enforcement training class and text, paying more attention to the back and forth communication than you do to the instructor is disrespectful and stupid (setting you up for missing the class or lecture). Even worse is to read a text in front of a firearms instructor trying to help you shoot better. Reading or sending non-work related texts while driving is just asking for trouble of the vehicle crash kind.
Of course the modern police officer is forced to operate the control, communications and safety equipment of the patrol vehicle on a daily basis. Many of these functions can be distracting to the primary function of driving but can be managed. Engaging in non-work related text messaging while on the job exposes you and innocent citizens to unacceptable risk. Technology should aid and assist you in your job and not expose you to greater risk. Texting while engaging in any call that requires your attention is reckless and especially so while driving - don't do it or pull over and do it safely.