Just because they aren't safe doesn't mean they aren't significantly safer then the use of handguns. Your restriction of them to purely deadly force situations eliminates an alternative intermediate step that can save lives, whether it be suspects, bystanders, or officers.
Of course they should never be used without proper situational awareness and judgment and the knowledge that they can be potentially fatal, but that doesn't mean they should be taken to the opposite extreme of being regarded as always being equally dangerous to handguns and therefore not used except in deadly force situations.
Tasers have been shown to be potentially deadly weapons. The Houston Chronicle, for instance, recorded the first
1,000 uses by police officers in Houston. In 95% of the cases, deadly force would not have been warranted. Over half of the cases came from routine calls - traffic stops, reports of "suspicious" people etc. In 350 of the cases, no arrest was made and no criminal charges filed.
These numbers do not suggest to me that the use of a Taser was warranted or that they are being taken as seriously by those using them as they should be.