In Morrowind any creature would just fall over or jump backwards when you gave it the final blow. That didn't spoil replayability for me, so I doubt if adding variable finishers will.
Indeed i agree there, but morrowind was much less reliant on visual action, when Oblivion came along with its rag-doll i was amazed, the action part of the rpg was boosted greatly by rag-doll, now its possible that rag-doll may be adversely affected by finishers in Skyrim.
Replayability? I initially thought finishers would detract from feeling 'I killed that guy' and give a more 'I wanted to do it myself' feeling. But really, you're going to kill that guy in the time that the finisher would take, so really, what's the big deal?
And rag-doll physics? You use a finisher, the guy drops to rag-doll physics - simple.
The rag-doll drop from a finisher is always just a straightforward slump to the floor, wheras it could have been a funny BONG! to the head from your mace and then roll over backwards with ragdoll, the finisher however always guarentees a boring slump to the floor once it is over.
Finisher --> then ALWAYS slump to the floor
Ragdoll --> less immediately spectacular but more chance of something interesting happening.