I never really cared for this concept. Yes, I know it was a gimmick to bring Spock back, but the implications are staggering: If the Katra truly exists, then how could any Vulcan within proximity of a Katra host ever die?
Seems like all the Vulcans would be seeking immortality by finding a Katra host only to be reintegrated into either a fresh body or someone elses!
But the movie made it clear that the
fal tor pan refusion of a
katra with a living body had "not been performed since ancient times, and then only in legend." It established that this was at best an extremely difficult procedure, only possible in the rarest of circumstances.
After all, we saw what happens when a
katra is placed in a body that already possesses an independent personality (McCoy's, in this case): the two minds struggle for control, endangering the sanity of the host. It couldn't work in the long term. The only way a
katra could be successfully resurrected in a living body is if that body were mindless, devoid of a functioning personality of its own. How often is that going to happen? Spock's situation was unique.
And even if some kind of cloning process could be used to create new bodies, it still wouldn't be a sure thing; the movie dealt with that as well by making it clear that the procedure was dangerous to the participants and not guaranteed to succeed. It required a highly adept Vulcan Master to work for a whole night to get it right, and even then it was imperfect, since Spock needed months of re-education afterward. There aren't enough top-level Masters on the planet to resurrect everyone even if it were a safe and reliable process.