And, OTOH, getting hit by a kill beam may be survivable to a degree. Your body suddenly sporting a charred hole somewhere doesn't necessarily tell whether there's a shallow pit there, or a fairly clean hole, or perhaps a huge cavity. Hence our doctor heroes sometimes just shaking their heads at such a wound, yet sometimes springing to action and even achieving results.
It's probably much like gunshot: the hit may mess you up inside real bad even if the puncture wound is insignificant. And in battlefield conditions, you only want to mess up a pound of flesh inside your opponent, to save on batteries, whereas in a peacetime adventure (such as all the TOS ones save for "Errand of Mercy" where our heroes went covert, and "Omega Glory" where the battlefield indeed got littered with bodies, even if off screen), you show mercy and vaporize the whole victim even though this drains your gun's reserves.