If you want a "hack and slash" game, why are you playing a RPG? Finishing moves are a "first person" player skill mechanism, not something that works well in a RPG where the character's skill should make some difference. There are dozens of FPS type games already, why corrupt nearly the only decent RPG left?
I disagree that finishing moves are player skill related, they are simply an animation that runs when you deal the final blow, I know you're probably thinking of that mortal combat fatality stuff, where you have to press a complex button combination, but I would hate that too, I would rather have finishing moves depend on where the death blow strikes, or maybe part of a critical hit?
I could see it work well in an RPG.
Personally, I want a system where, if the hit that takes out the last health in the health bar, is not a strike to the torso or head, then I would rather have the enemy fall to the ground and scream in pain, or some other neutralizing state, which doesn't involve immediate death, then you can execute them, and depending on the weapon, area of attack, and enemy type, you get a different finishing move.
Of course there should also be some kind of "finishing move" that you don't have to take them down first to achieve, an example of something like that could be if you kill someone with a power attack, like if you kill someone with a hard forward thrust, then you actually run them through.