They generally are, as they are meant to be the step before lethal force is used. The problem with tasers is the same as with pepper spray though: In many situations, particularly when the attacker is under the influence of drugs, they won't be enough to stop the violence in progress. At best they are a temporary distraction but won't be enough to subdue attacker in the long term. They will just keep attacking without seeing or resume attacking as soon as the spasms subside. In some cases that respite will be enough to let the police get the upper hand and restrain the attacker, but it is by no means given and failing to do so increases the risk of the victim, bystanders or the police getting injured. Hence the "safe bet" becomes to escalate straight to lethal force, because a lethally injured or dead attacker will not be able to pose a further threat.
I realize I'm coming off as defending bad police practices here and I want to make clear that a lot of police shootings in the US are entirely unjustified as the police responds way too aggressively to minor provocations or, as in the case with Trayvon Martin, just pre-emptively uses their gun when there's no justifiable threat to the police. But in violent situations, when you're sympathetic nervous system is firing like crazy and people might be seriously injured or killed it is often really hard, if not downright impossible, to accurately judge the right amount of force to subdue someone without causing unnecessary injury. It is guess work and often that means using more force then necessary as a precaution.
