A tazer is an excellent tool to obtain compliance with someone without having to use physical force, which very commonly results in injury.
Yes, there are tazer deaths, and they are investigated. The majority of tazer deaths result when either the tazer is used too many times consecutively on a subject (bad bad police man!) or when the subject is on many drugs and the strains already on their heart and increased by the use of the tazer (OMG Those police are supposed to already know what level of drugs are in someone right?!?!? ...stupid public).
In the long run, zapping someone with a tazer (i like the drive stun where you pull off the projectile end and just zap em for a second like an old "stun gun") usually guarantees that they will comply and you will be capable of getting them secured.
Of course, our general public will continue to talk about tazers like they know what the **** they are talking about.