Yeah it's a waste of time in my opinion because the game further promotes wasting time which rewards people for wasting even more time. -_- If something is going to die, it's going to die. "I'll hit you harder so you drop more loot, you living pinata!" It makes 100% sense in games like Mortal Kombat for me though, when the game is all about beating everything into a bloody pulp. xD
I can't find any satisfaction or point in pummeling a corpse and being rewarded for it, but hey, that's just me. :)
I still entirely disagree with your opinion.
In Final Fantasy X, when an Overkill happens it doesn't add any additional time to the game at all. You get a little text informing that an Overkill occurred as the damage appears and the reward appears in the loot list with no additional fanfare.
If you play the game normally, you may benefit from the odd Overkill, but you can outright ignore the feature entirely.
If you're a hard-core player then you're going to want to invest the extra time into setting up Overkills and gaining the bonus drops; that isn't time wasted, that's time that a hard-core player would have spent anyway.
What is a nice-little bonus with no-drawbacks at all for regular players becomes an additional level for hard-core players to play the game. That's exactly what RPGs need for longevity - it needs to be simple, no-frills for a regular run-through and for additional runs to have re-playability with the hard-core grinding (which is entirely optional - reminder).
If you feel the rewards for Overkills takes too much time, then don't worry about them. They aren't detracting from your experience (they add to your experience when they do happen every now and then by accident).
Additionally, these are RPGs we're talking about; there is no pummelling to a pulp occurring, the enemy still drops on the ground with standard animations and then fades away with a small boom.
I think your complaint of "time wasted" is entirely unwarranted and ill-considered...